Network Working Group                                   J. Klensin
Internet-Draft                                   December 14, 2008
Obsoletes: 5321 (if approved)
Intended status: Standards Track


                       Simple Mail Transfer Protocol
                   draft-klensin-rfc5321-numbered-00.txt

  Status of This Memo

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   This Internet-Draft will expire on June 17, 2009.

Abstract

     This document is a specification of the basic protocol for Internet
     electronic mail transport.  It consolidates, updates, and clarifies
     several previous documents, making all or parts of most of them
     obsolete.  It covers the SMTP extension mechanisms and best practices
     for the contemporary Internet, but does not provide details about
     particular extensions.  Although SMTP was designed as a mail
     transport and delivery protocol, this specification also contains
     information that is important to its use as a "mail submission"
     protocol for "split-UA" (User Agent) mail reading systems and mobile
     environments.

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  Internet-Draft        RFC5321-Numbered               Dec 2008


   Table of Contents

   [[To be supplied or see RFC 5321]]


        [[Note in Draft: This special Internet-Draft version of RFC 5321 is
        supplied with line numbers to facilitate the possibility that the
        modifications to bring the document up to (full) Internet Standard
        may have to be done as deltas because it is unlikely the
        authorization for the additional rights required by RFC 5378 can be
        obtained from the authors of all of the text included in 5321.
        Should that concern be unjustified, this document will be discarded.

    This Internet-Draft has been paginated but is not page numbered (the
        page number on the last page is one of several hacks to make the
        automatic posting tool work), only line-numnbered, and the Table of
        Contents has been eliminated. The line numbering is somewhat
        artificial and has also been hacked to get past the automatic
        posting tool but, after the introductory material, reflects actual
        line numbers in RFC 5321.]]



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  1.  Introduction
226
  1.1.  Transport of Electronic Mail
228
     The objective of the Simple Mail Transfer Protocol (SMTP) is to
     transfer mail reliably and efficiently.
231
     SMTP is independent of the particular transmission subsystem and
     requires only a reliable ordered data stream channel.  While this
     document specifically discusses transport over TCP, other transports
     are possible.  Appendices to RFC 821 [1] describe some of them.
236
     An important feature of SMTP is its capability to transport mail
     across multiple networks, usually referred to as "SMTP mail relaying"
     (see Section 3.6).  A network consists of the mutually-TCP-accessible
     hosts on the public Internet, the mutually-TCP-accessible hosts on a
     firewall-isolated TCP/IP Intranet, or hosts in some other LAN or WAN
     environment utilizing a non-TCP transport-level protocol.  Using
     SMTP, a process can transfer mail to another process on the same
     network or to some other network via a relay or gateway process
     accessible to both networks.
246
     In this way, a mail message may pass through a number of intermediate
     relay or gateway hosts on its path from sender to ultimate recipient.
     The Mail eXchanger mechanisms of the domain name system (RFC 1035
     [2], RFC 974 [12], and Section 5 of this document) are used to
     identify the appropriate next-hop destination for a message being
     transported.
253
  1.2.  History and Context for This Document
255
     This document is a specification of the basic protocol for the
     Internet electronic mail transport.  It consolidates, updates and
     clarifies, but does not add new or change existing functionality of
     the following:
260
     o  the original SMTP (Simple Mail Transfer Protocol) specification of
        RFC 821 [1],
263
     o  domain name system requirements and implications for mail
        transport from RFC 1035 [2] and RFC 974 [12],
266
     o  the clarifications and applicability statements in RFC 1123 [3],
        and
269


     o  material drawn from the SMTP Extension mechanisms in RFC 1869
        [13].
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     o  Editorial and clarification changes to RFC 2821 [14] to bring that
        specification to Draft Standard.
283
     It obsoletes RFC 821, RFC 974, RFC 1869, and RFC 2821 and updatesRFC
     1123 (replacing the mail transport materials of RFC 1123).  However,
     RFC 821 specifies some features that were not in significant use in
     the Internet by the mid-1990s and (in appendices) some additional
     transport models.  Those sections are omitted here in the interest of
     clarity and brevity; readers needing them should refer to RFC 821.
290
     It also includes some additional material from RFC 1123 that required
     amplification.  This material has been identified in multiple ways,
     mostly by tracking flaming on various lists and newsgroups and
     problems of unusual readings or interpretations that have appeared as
     the SMTP extensions have been deployed.  Where this specification
     moves beyond consolidation and actually differs from earlier
     documents, it supersedes them technically as well as textually.
298
     Although SMTP was designed as a mail transport and delivery protocol,
     this specification also contains information that is important toits
     use as a "mail submission" protocol, as recommended for Post Office
     Protocol (POP) (RFC 937 [15], RFC 1939 [16]) and IMAP (RFC 3501
     [17]).  In general, the separate mail submission protocol specified
     in RFC 4409 [18] is now preferred to direct use of SMTP; more
     discussion of that subject appears in that document.
306
     Section 2.3 provides definitions of terms specific to this document.
     Except when the historical terminology is necessary for clarity, this
     document uses the current 'client' and 'server' terminology to
     identify the sending and receiving SMTP processes, respectively.
311
     A companion document, RFC 5322 [4], discusses message header sections
     and bodies and specifies formats and structures for them.
314
  1.3.  Document Conventions
316
     The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
     "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in t


his
     document are to be interpreted as described in RFC 2119 [5].  As each
     of these terms was intentionally and carefully chosen to improve the
     interoperability of email, each use of these terms is to be treated
     as a conformance requirement.
323
     Because this document has a long history and to avoid the risk of
     various errors and of confusing readers and documents that point to
     this one, most examples and the domain names they contain are
     preserved from RFC 2821.  Readers are cautioned that these are
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     illustrative examples that should not actually be used in either code
     or configuration files.
339
  2.  The SMTP Model
341
  2.1.  Basic Structure
343
     The SMTP design can be pictured as:
345
                    +----------+                +----------+
        +------+    |          |                |          |
        | User |<-->|          |      SMTP      |          |
        +------+    |  Client- |Commands/Replies| Server-  |
        +------+    |   SMTP   |<-------------->|    SMTP  |    +------+
        | File |<-->|          |    and Mail    |          |<-->| File|
        |System|    |          |                |          |    |System|
        +------+    +----------+                +----------+    +------+
                     SMTP client                SMTP server
355
     When an SMTP client has a message to transmit, it establishes a two-
     way transmission channel to an SMTP server.  The responsibility of an
     SMTP client is to transfer mail messages to one or more SMTP servers,
     or report its failure to do so.
360
     The means by which a mail message is presented to an SMTP client,and
     how that client determines the identifier(s) ("names") of the
     domain(s) to which mail messages are to be transferred, is a local
     matter, and is not addressed by this document.  In some cases, the
     designated domain(s), or those determined by an SMTP client, will
     identify the final destination(s) of the mail message.  In other
     cases, common with SMTP clients associated with implementations of
     the POP (RFC 937 [15], RFC 1939 [16]) or IMAP (RFC 3501 [17])
     protocols, or when the SMTP client is inside an isolated transport
     service environment, the domain determined will identify an
     intermediate destination through which all mail messages are to be
     relayed.  SMTP clients that transfer all traffic regardless of the
     target domains associated with the individual messages, or that do
     not maintain queues for retrying message transmissions that initially


     cannot be completed, may otherwise conform to this specification but
     are not considered fully-capable.  Fully-capable SMTP
     implementations, including the relays used by these less capable
     ones, and their destinations, are expected to support all of the
     queuing, retrying, and alternate address functions discussed in this
     specification.  In many situations and configurations, the less-
     capable clients discussed above SHOULD be using the message
     submission protocol (RFC 4409 [18]) rather than SMTP.
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     The means by which an SMTP client, once it has determined a target
     domain, determines the identity of an SMTP server to which a copyof
     a message is to be transferred, and then performs that transfer, is
     covered by this document.  To effect a mail transfer to an SMTP
     server, an SMTP client establishes a two-way transmission channelto
     that SMTP server.  An SMTP client determines the address of an
     appropriate host running an SMTP server by resolving a destination
     domain name to either an intermediate Mail eXchanger host or a final
     target host.
402
     An SMTP server may be either the ultimate destination or an
     intermediate "relay" (that is, it may assume the role of an SMTP
     client after receiving the message) or "gateway" (that is, it may
     transport the message further using some protocol other than SMTP).
     SMTP commands are generated by the SMTP client and sent to the SMTP
     server.  SMTP replies are sent from the SMTP server to the SMTP
     client in response to the commands.
410
     In other words, message transfer can occur in a single connection
     between the original SMTP-sender and the final SMTP-recipient, orcan
     occur in a series of hops through intermediary systems.  In either
     case, once the server has issued a success response at the end ofthe
     mail data, a formal handoff of responsibility for the message occurs:
     the protocol requires that a server MUST accept responsibility for
     either delivering the message or properly reporting the failure to do
     so (see Sections 6.1, 6.2, and 7.8, below).
419
     Once the transmission channel is established and initial handshaking
     is completed, the SMTP client normally initiates a mail transaction.
     Such a transaction consists of a series of commands to specify the
     originator and destination of the mail and transmission of the
     message content (including any lines in the header section or other
     structure) itself.  When the same message is sent to multiple
     recipients, this protocol encourages the transmission of only one
     copy of the data for all recipients at the same destination (or
     intermediate relay) host.
429


     The server responds to each command with a reply; replies may
     indicate that the command was accepted, that additional commands are
     expected, or that a temporary or permanent error condition exists.
     Commands specifying the sender or recipients may include server-
     permitted SMTP service extension requests, as discussed in
     Section 2.2.  The dialog is purposely lock-step, one-at-a-time,
     although this can be modified by mutually agreed upon extension
     requests such as command pipelining (RFC 2920 [19]).
438
     Once a given mail message has been transmitted, the client may either
     request that the connection be shut down or may initiate other mail
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     transactions.  In addition, an SMTP client may use a connection to an
     SMTP server for ancillary services such as verification of email
     addresses or retrieval of mailing list subscriber addresses.
452
     As suggested above, this protocol provides mechanisms for the
     transmission of mail.  Historically, this transmission normally
     occurred directly from the sending user's host to the receiving
     user's host when the two hosts are connected to the same transport
     service.  When they are not connected to the same transport service,
     transmission occurs via one or more relay SMTP servers.  A very
     common case in the Internet today involves submission of the original
     message to an intermediate, "message submission" server, which is
     similar to a relay but has some additional properties; such servers
     are discussed in Section 2.3.10 and at some length in RFC 4409 [18].
     An intermediate host that acts as either an SMTP relay or as a
     gateway into some other transmission environment is usually selected
     through the use of the domain name service (DNS) Mail eXchanger
     mechanism.
467
     Usually, intermediate hosts are determined via the DNS MX record,not
     by explicit "source" routing (see Section 5 and Appendix C and
     Appendix F.2).
471
  2.2.  The Extension Model
473
  2.2.1.  Background
475
     In an effort that started in 1990, approximately a decade after RFC
     821 was completed, the protocol was modified with a "service
     extensions" model that permits the client and server to agree to
     utilize shared functionality beyond the original SMTP requirements.
     The SMTP extension mechanism defines a means whereby an extended SMTP
     client and server may recognize each other, and the server can inform
     the client as to the service extensions that it supports.
483
     Contemporary SMTP implementations MUST support the basic extension
     mechanisms.  For instance, servers MUST support the EHLO command even
     if they do not implement any specific extensions and clients SHOULD
     preferentially utilize EHLO rather than HELO.  (However, for
     compatibility with older conforming implementations, SMTP clients


and
     servers MUST support the original HELO mechanisms as a fallback.)
     Unless the different characteristics of HELO must be identified for
     interoperability purposes, this document discusses only EHLO.
492
     SMTP is widely deployed and high-quality implementations have proven
     to be very robust.  However, the Internet community now considers
     some services to be important that were not anticipated when the
     protocol was first designed.  If support for those services is tobe
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     added, it must be done in a way that permits older implementations to
     continue working acceptably.  The extension framework consists of:
507
     o  The SMTP command EHLO, superseding the earlier HELO,
509
     o  a registry of SMTP service extensions,
511
     o  additional parameters to the SMTP MAIL and RCPT commands, and
513
     o  optional replacements for commands defined in this protocol, such
        as for DATA in non-ASCII transmissions (RFC 3030 [20]).
516
     SMTP's strength comes primarily from its simplicity.  Experience with
     many protocols has shown that protocols with few options tend towards
     ubiquity, whereas protocols with many options tend towards obscurity.
520
     Each and every extension, regardless of its benefits, must be
     carefully scrutinized with respect to its implementation, deployment,
     and interoperability costs.  In many cases, the cost of extendingthe
     SMTP service will likely outweigh the benefit.
525
  2.2.2.  Definition and Registration of Extensions
527
     The IANA maintains a registry of SMTP service extensions.  A
     corresponding EHLO keyword value is associated with each extension.
     Each service extension registered with the IANA must be defined in a
     formal Standards-Track or IESG-approved Experimental protocol
     document.  The definition must include:
533
     o  the textual name of the SMTP service extension;
535
     o  the EHLO keyword value associated with the extension;
537
     o  the syntax and possible values of parameters associated with the
        EHLO keyword value;
540
     o  any additional SMTP verbs associated with the extension
        (additional verbs will usually be, but are not required to be,the
        same as the EHLO keyword value);
544
     o  any new parameters the extension associates with the MAIL or RCPT
        verbs;


547
     o  a description of how support for the extension affects the
        behavior of a server and client SMTP; and




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     o  the increment by which the extension is increasing the maximum
        length of the commands MAIL and/or RCPT, over that specified in
        this Standard.
564
     In addition, any EHLO keyword value starting with an upper or lower
     case "X" refers to a local SMTP service extension used exclusively
     through bilateral agreement.  Keywords beginning with "X" MUST NOT be
     used in a registered service extension.  Conversely, keyword values
     presented in the EHLO response that do not begin with "X" MUST
     correspond to a Standard, Standards-Track, or IESG-approved
     Experimental SMTP service extension registered with IANA.  A
     conforming server MUST NOT offer non-"X"-prefixed keyword values that
     are not described in a registered extension.
574
     Additional verbs and parameter names are bound by the same rules as
     EHLO keywords; specifically, verbs beginning with "X" are local
     extensions that may not be registered or standardized.  Conversely,
     verbs not beginning with "X" must always be registered.
579
  2.2.3.  Special Issues with Extensions
581
     Extensions that change fairly basic properties of SMTP operation are
     permitted.  The text in other sections of this document must be
     understood in that context.  In particular, extensions can changethe
     minimum limits specified in Section 4.5.3, can change the ASCII
     character set requirement as mentioned above, or can introduce some
     optional modes of message handling.
588
     In particular, if an extension implies that the delivery path
     normally supports special features of that extension, and an
     intermediate SMTP system finds a next hop that does not support the
     required extension, it MAY choose, based on the specific extension
     and circumstances, to requeue the message and try later and/or try an
     alternate MX host.  If this strategy is employed, the timeout to fall
     back to an unextended format (if one is available) SHOULD be less
     than the normal timeout for bouncing as undeliverable (e.g., if
     normal timeout is three days, the requeue timeout before attempting
     to transmit the mail without the extension might be one day).
599


  2.3.  SMTP Terminology
601
  2.3.1.  Mail Objects
603
     SMTP transports a mail object.  A mail object contains an envelope
     and content.
606
     The SMTP envelope is sent as a series of SMTP protocol units
     (described in Section 3).  It consists of an originator address (to
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     which error reports should be directed), one or more recipient
     addresses, and optional protocol extension material.  Historically,
     variations on the reverse-path (originator) address specification
     command (MAIL) could be used to specify alternate delivery modes,
     such as immediate display; those variations have now been deprecated
     (see Appendix F and Appendix F.6).
623
     The SMTP content is sent in the SMTP DATA protocol unit and has two
     parts: the header section and the body.  If the content conforms to
     other contemporary standards, the header section consists of a
     collection of header fields, each consisting of a header name, a
     colon, and data, structured as in the message format specification
     (RFC 5322 [4]); the body, if structured, is defined according to MIME
     (RFC 2045 [21]).  The content is textual in nature, expressed using
     the US-ASCII repertoire [6].  Although SMTP extensions (such as
     "8BITMIME", RFC 1652 [22]) may relax this restriction for the content
     body, the content header fields are always encoded using the US-ASCII
     repertoire.  Two MIME extensions (RFC 2047 [23] and RFC 2231 [24])
     define an algorithm for representing header values outside the US-
     ASCII repertoire, while still encoding them using the US-ASCII
     repertoire.
638
  2.3.2.  Senders and Receivers
640
     In RFC 821, the two hosts participating in an SMTP transaction were
     described as the "SMTP-sender" and "SMTP-receiver".  This document
     has been changed to reflect current industry terminology and hence
     refers to them as the "SMTP client" (or sometimes just "the client")
     and "SMTP server" (or just "the server"), respectively.  Since a
     given host may act both as server and client in a relay situation,
     "receiver" and "sender" terminology is still used where needed for
     clarity.
649
  2.3.3.  Mail Agents and Message Stores
651
     Additional mail system terminology became common after RFC 821 was
     published and, where convenient, is used in this specification.


In
     particular, SMTP servers and clients provide a mail transport service
     and therefore act as "Mail Transfer Agents" (MTAs).  "Mail User
     Agents" (MUAs or UAs) are normally thought of as the sources and
     targets of mail.  At the source, an MUA might collect mail to be
     transmitted from a user and hand it off to an MTA; the final
     ("delivery") MTA would be thought of as handing the mail off to an
     MUA (or at least transferring responsibility to it, e.g., by
     depositing the message in a "message store").  However, while these
     terms are used with at least the appearance of great precision in
     other environments, the implied boundaries between MUAs and MTAs
     often do not accurately match common, and conforming, practices with
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     Internet mail.  Hence, the reader should be cautious about inferring
     the strong relationships and responsibilities that might be implied
     if these terms were used elsewhere.
676
  2.3.4.  Host
678
     For the purposes of this specification, a host is a computer system
     attached to the Internet (or, in some cases, to a private TCP/IP
     network) and supporting the SMTP protocol.  Hosts are known by names
     (see the next section); they SHOULD NOT be identified by numerical
     addresses, i.e., by address literals as described in Section 4.1.2.
684
  2.3.5.  Domain Names
686
     A domain name (or often just a "domain") consists of one or more
     components, separated by dots if more than one appears.  In the case
     of a top-level domain used by itself in an email address, a single
     string is used without any dots.  This makes the requirement,
     described in more detail below, that only fully-qualified domain
     names appear in SMTP transactions on the public Internet,
     particularly important where top-level domains are involved.  These
     components ("labels" in DNS terminology, RFC 1035 [2]) are restricted
     for SMTP purposes to consist of a sequence of letters, digits, and
     hyphens drawn from the ASCII character set [6].  Domain names are
     used as names of hosts and of other entities in the domain name
     hierarchy.  For example, a domain may refer to an alias (label ofa
     CNAME RR) or the label of Mail eXchanger records to be used to
     deliver mail instead of representing a host name.  See RFC 1035 [2]
     and Section 5 of this specification.
702
     The domain name, as described in this document and in RFC 1035 [2],
     is the entire, fully-qualified name (often referred to as an "FQDN").
     A domain name that is not in FQDN form is no more than a local alias.
     Local aliases MUST NOT appear in any SMTP transaction.
707
     Only resolvable, fully-qualified domain names (FQDNs) are permitted
     when domain names are used in SMTP.  In other words, names that can


     be resolved to MX RRs or address (i.e., A or AAAA) RRs (as discussed
     in Section 5) are permitted, as are CNAME RRs whose targets can be
     resolved, in turn, to MX or address RRs.  Local nicknames or
     unqualified names MUST NOT be used.  There are two exceptions to the
     rule requiring FQDNs:
715
     o  The domain name given in the EHLO command MUST be either a primary
        host name (a domain name that resolves to an address RR) or, if
        the host has no name, an address literal, as described in
        Section 4.1.3 and discussed further in the EHLO discussion of
        Section 4.1.4.
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     o  The reserved mailbox name "postmaster" may be used in a RCPT
        command without domain qualification (see Section 4.1.1.3) and
        MUST be accepted if so used.
732
  2.3.6.  Buffer and State Table
734
     SMTP sessions are stateful, with both parties carefully maintaining a
     common view of the current state.  In this document, we model this
     state by a virtual "buffer" and a "state table" on the server that
     may be used by the client to, for example, "clear the buffer" or
     "reset the state table", causing the information in the buffer tobe
     discarded and the state to be returned to some previous state.
741
  2.3.7.  Commands and Replies
743
     SMTP commands and, unless altered by a service extension, message
     data, are transmitted from the sender to the receiver via the
     transmission channel in "lines".
747
     An SMTP reply is an acknowledgment (positive or negative) sent in
     "lines" from receiver to sender via the transmission channel in
     response to a command.  The general form of a reply is a numeric
     completion code (indicating failure or success) usually followed by a
     text string.  The codes are for use by programs and the text is
     usually intended for human users.  RFC 3463 [25], specifies further
     structuring of the reply strings, including the use of supplemental
     and more specific completion codes (see also RFC 5248 [26]).
756
  2.3.8.  Lines
758
     Lines consist of zero or more data characters terminated by the
     sequence ASCII character "CR" (hex value 0D) followed immediatelyby
     ASCII character "LF" (hex value 0A).  This termination sequence is
     denoted as <CRLF> in this document.  Conforming implementations MUST
     NOT recognize or generate any other character or character sequence
     as a line terminator.  Limits MAY be imposed on line lengths by
     servers (see Section 4).
766
     In addition, the appearance of "bare" "CR" or "LF" characters in text
     (i.e., either without the other) has a long history of causing
     problems in mail implementations and applications that use the mail
     system as a tool.  SMTP client implementations MUST NOT transmit


     these characters except when they are intended as line terminators
     and then MUST, as indicated above, transmit them only as a <CRLF>
     sequence.
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  2.3.9.  Message Content and Mail Data
786
     The terms "message content" and "mail data" are used interchangeably
     in this document to describe the material transmitted after the DATA
     command is accepted and before the end of data indication is
     transmitted.  Message content includes the message header sectionand
     the possibly structured message body.  The MIME specification (RFC
     2045 [21]) provides the standard mechanisms for structured message
     bodies.
794
  2.3.10.  Originator, Delivery, Relay, and Gateway Systems
796
     This specification makes a distinction among four types of SMTP
     systems, based on the role those systems play in transmitting
     electronic mail.  An "originating" system (sometimes called an SMTP
     originator) introduces mail into the Internet or, more generally,
     into a transport service environment.  A "delivery" SMTP system is
     one that receives mail from a transport service environment and
     passes it to a mail user agent or deposits it in a message store that
     a mail user agent is expected to subsequently access.  A "relay" SMTP
     system (usually referred to just as a "relay") receives mail froman
     SMTP client and transmits it, without modification to the message
     data other than adding trace information, to another SMTP server for
     further relaying or for delivery.
809
     A "gateway" SMTP system (usually referred to just as a "gateway")
     receives mail from a client system in one transport environment and
     transmits it to a server system in another transport environment.
     Differences in protocols or message semantics between the transport
     environments on either side of a gateway may require that the gateway
     system perform transformations to the message that are not permitted
     to SMTP relay systems.  For the purposes of this specification,
     firewalls that rewrite addresses should be considered as gateways,
     even if SMTP is used on both sides of them (see RFC 2979 [27]).
819
  2.3.11.  Mailbox and Address
821
     As used in this specification, an "address" is a character string
     that identifies a user to whom mail will be sent or a location in


to
     which mail will be deposited.  The term "mailbox" refers to that
     depository.  The two terms are typically used interchangeably unless
     the distinction between the location in which mail is placed (the
     mailbox) and a reference to it (the address) is important.  An
     address normally consists of user and domain specifications.  The
     standard mailbox naming convention is defined to be
     "local-part@domain"; contemporary usage permits a much broader set of
     applications than simple "user names".  Consequently, and due to a
     long history of problems when intermediate hosts have attempted to
833
834
835

837











































  Internet-Draft        RFC5321-Numbered               Dec 2008
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840
     optimize transport by modifying them, the local-part MUST be
     interpreted and assigned semantics only by the host specified in the
     domain part of the address.
844
  2.4.  General Syntax Principles and Transaction Model
846
     SMTP commands and replies have a rigid syntax.  All commands begin
     with a command verb.  All replies begin with a three digit numeric
     code.  In some commands and replies, arguments are required following
     the verb or reply code.  Some commands do not accept arguments (after
     the verb), and some reply codes are followed, sometimes optionally,
     by free form text.  In both cases, where text appears, it is
     separated from the verb or reply code by a space character.  Complete
     definitions of commands and replies appear in Section 4.
855
     Verbs and argument values (e.g., "TO:" or "to:" in the RCPT command
     and extension name keywords) are not case sensitive, with the sole
     exception in this specification of a mailbox local-part (SMTP
     Extensions may explicitly specify case-sensitive elements).  Thatis,
     a command verb, an argument value other than a mailbox local-part,
     and free form text MAY be encoded in upper case, lower case, or any
     mixture of upper and lower case with no impact on its meaning.  The
     local-part of a mailbox MUST BE treated as case sensitive.
     Therefore, SMTP implementations MUST take care to preserve the case
     of mailbox local-parts.  In particular, for some hosts, the user
     "smith" is different from the user "Smith".  However, exploiting the
     case sensitivity of mailbox local-parts impedes interoperability and
     is discouraged.  Mailbox domains follow normal DNS rules and are
     hence not case sensitive.
870
     A few SMTP servers, in violation of this specification (and RFC 821)
     require that command verbs be encoded by clients in upper case.
     Implementations MAY wish to employ this encoding to accommodate those
     servers.
875
     The argument clause consists of a variable-length character string


     ending with the end of the line, i.e., with the character sequence
     <CRLF>.  The receiver will take no action until this sequence is
     received.
880
     The syntax for each command is shown with the discussion of that
     command.  Common elements and parameters are shown in Section 4.1.2.
883
     Commands and replies are composed of characters from the ASCII
     character set [6].  When the transport service provides an 8-bit byte
     (octet) transmission channel, each 7-bit character is transmitted,
     right justified, in an octet with the high-order bit cleared to zero.
     More specifically, the unextended SMTP service provides 7-bit
889
890
891

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  Internet-Draft        RFC5321-Numbered               Dec 2008
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896
     transport only.  An originating SMTP client that has not successfully
     negotiated an appropriate extension with a particular server (seethe
     next paragraph) MUST NOT transmit messages with information in the
     high-order bit of octets.  If such messages are transmitted in
     violation of this rule, receiving SMTP servers MAY clear the high-
     order bit or reject the message as invalid.  In general, a relay SMTP
     SHOULD assume that the message content it has received is valid and,
     assuming that the envelope permits doing so, relay it without
     inspecting that content.  Of course, if the content is mislabeledand
     the data path cannot accept the actual content, this may result in
     the ultimate delivery of a severely garbled message to the recipient.
     Delivery SMTP systems MAY reject such messages, or return them as
     undeliverable, rather than deliver them.  In the absence of a server-
     offered extension explicitly permitting it, a sending SMTP systemis
     not permitted to send envelope commands in any character set other
     than US-ASCII.  Receiving systems SHOULD reject such commands,
     normally using "500 syntax error - invalid character" replies.
914
     8-bit message content transmission MAY be requested of the serverby
     a client using extended SMTP facilities, notably the "8BITMIME"
     extension, RFC 1652 [22]. 8BITMIME SHOULD be supported by SMTP
     servers.  However, it MUST NOT be construed as authorization to
     transmit unrestricted 8-bit material, nor does 8BITMIME authorize
     transmission of any envelope material in other than ASCII. 8BITMIME
     MUST NOT be requested by senders for material with the high bit on
     that is not in MIME format with an appropriate content-transfer
     encoding; servers MAY reject such messages.
924
     The metalinguistic notation used in this document corresponds to the
     "Augmented BNF" used in other Internet mail system documents.  The
     reader who is not familiar with that syntax should consult the ABNF
     specification in RFC 5234 [7].  Metalanguage terms used in running
     text are surrounded by pointed brackets (e.g., <CRLF>) for clarity.
     The reader is cautioned that the grammar expressed in the
     metalanguage is not comprehensive.  There are many instances in w


hich
     provisions in the text constrain or otherwise modify the syntax or
     semantics implied by the grammar.
934
  3.  The SMTP Procedures: An Overview
936
     This section contains descriptions of the procedures used in SMTP:
     session initiation, mail transaction, forwarding mail, verifying
     mailbox names and expanding mailing lists, and opening and closing
     exchanges.  Comments on relaying, a note on mail domains, and a
     discussion of changing roles are included at the end of this section.
     Several complete scenarios are presented in Appendix D.
943
944
945
946
947

949







































  Internet-Draft        RFC5321-Numbered               Dec 2008
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952
  3.1.  Session Initiation
954
     An SMTP session is initiated when a client opens a connection to a
     server and the server responds with an opening message.
957
     SMTP server implementations MAY include identification of their
     software and version information in the connection greeting reply
     after the 220 code, a practice that permits more efficient isolation
     and repair of any problems.  Implementations MAY make provision for
     SMTP servers to disable the software and version announcement where
     it causes security concerns.  While some systems also identify their
     contact point for mail problems, this is not a substitute for
     maintaining the required "postmaster" address (see Section 4).
966
     The SMTP protocol allows a server to formally reject a mail session
     while still allowing the initial connection as follows: a 554
     response MAY be given in the initial connection opening message
     instead of the 220.  A server taking this approach MUST still wait
     for the client to send a QUIT (see Section 4.1.1.10) before closing
     the connection and SHOULD respond to any intervening commands with
     "503 bad sequence of commands".  Since an attempt to make an SMTP
     connection to such a system is probably in error, a server returning
     a 554 response on connection opening SHOULD provide enough
     information in the reply text to facilitate debugging of the sending
     system.
978
  3.2.  Client Initiation
980
     Once the server has sent the greeting (welcoming) message and the
     client has received it, the client normally sends the EHLO command to
     the server, indicating the client's identity.  In addition to opening
     the session, use of EHLO indicates that the client is able to process
     service extensions and requests that the server provide a list ofthe
     extensions it supports.  Older SMTP systems that are unable to
     support service extensions, and contemporary clients that do not
     require service extensions in the mail session being initiated, MAY
     use HELO instead of EHLO.  Servers MUST NOT return the extended EHLO-
     style response to a HELO command.  For a particular connection


     attempt, if the server returns a "command not recognized" response to
     EHLO, the client SHOULD be able to fall back and send HELO.
993
     In the EHLO command, the host sending the command identifies itself;
     the command may be interpreted as saying "Hello, I am <domain>" (and,
     in the case of EHLO, "and I support service extension requests").
997
998
999
1000
1001
1002
1003

1005












































  Internet-Draft        RFC5321-Numbered               Dec 2008
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1008
  3.3.  Mail Transactions
1010
     There are three steps to SMTP mail transactions.  The transaction
     starts with a MAIL command that gives the sender identification. (In
     general, the MAIL command may be sent only when no mail transaction
     is in progress; see Section 4.1.4.)  A series of one or more RCPT
     commands follows, giving the receiver information.  Then, a DATA
     command initiates transfer of the mail data and is terminated by the
     "end of mail" data indicator, which also confirms the transaction.
1018
     The first step in the procedure is the MAIL command.
1020
        MAIL FROM:<reverse-path> [SP <mail-parameters> ] <CRLF>
1022
     This command tells the SMTP-receiver that a new mail transaction is
     starting and to reset all its state tables and buffers, includingany
     recipients or mail data.  The <reverse-path> portion of the firstor
     only argument contains the source mailbox (between "<" and ">"
     brackets), which can be used to report errors (see Section 4.2 for a
     discussion of error reporting).  If accepted, the SMTP server returns
     a "250 OK" reply.  If the mailbox specification is not acceptablefor
     some reason, the server MUST return a reply indicating whether the
     failure is permanent (i.e., will occur again if the client tries to
     send the same address again) or temporary (i.e., the address might be
     accepted if the client tries again later).  Despite the apparent
     scope of this requirement, there are circumstances in which the
     acceptability of the reverse-path may not be determined until oneor
     more forward-paths (in RCPT commands) can be examined.  In those
     cases, the server MAY reasonably accept the reverse-path (with a 250
     reply) and then report problems after the forward-paths are received
     and examined.  Normally, failures produce 550 or 553 replies.
1040
     Historically, the <reverse-path> was permitted to contain more than
     just a mailbox; however, contemporary systems SHOULD NOT use source
     routing (see Appendix C).
1044
     The optional <mail-parameters> are associated with negotiated SMT


P
     service extensions (see Section 2.2).
1047
     The second step in the procedure is the RCPT command.  This step of
     the procedure can be repeated any number of times.
1050
        RCPT TO:<forward-path> [ SP <rcpt-parameters> ] <CRLF>
1052
     The first or only argument to this command includes a forward-path
     (normally a mailbox and domain, always surrounded by "<" and ">"
     brackets) identifying one recipient.  If accepted, the SMTP server
     returns a "250 OK" reply and stores the forward-path.  If the
1057
1058
1059

1061










































  Internet-Draft        RFC5321-Numbered               Dec 2008
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1064
     recipient is known not to be a deliverable address, the SMTP server
     returns a 550 reply, typically with a string such as "no such user -
     " and the mailbox name (other circumstances and reply codes are
     possible).
1069
     The <forward-path> can contain more than just a mailbox.
     Historically, the <forward-path> was permitted to contain a source
     routing list of hosts and the destination mailbox; however,
     contemporary SMTP clients SHOULD NOT utilize source routes (see
     Appendix C).  Servers MUST be prepared to encounter a list of source
     routes in the forward-path, but they SHOULD ignore the routes or MAY
     decline to support the relaying they imply.  Similarly, servers MAY
     decline to accept mail that is destined for other hosts or systems.
     These restrictions make a server useless as a relay for clients that
     do not support full SMTP functionality.  Consequently, restricted-
     capability clients MUST NOT assume that any SMTP server on the
     Internet can be used as their mail processing (relaying) site.  If a
     RCPT command appears without a previous MAIL command, the server MUST
     return a 503 "Bad sequence of commands" response.  The optional
     <rcpt-parameters> are associated with negotiated SMTP service
     extensions (see Section 2.2).
1086
     Since it has been a common source of errors, it is worth noting that
     spaces are not permitted on either side of the colon following FROM
     in the MAIL command or TO in the RCPT command.  The syntax is exactly
     as given above.
1091
     The third step in the procedure is the DATA command (or some
     alternative specified in a service extension).
1094
        DATA <CRLF>
1096
     If accepted, the SMTP server returns a 354 Intermediate reply and
     considers all succeeding lines up to but not including the end of
     mail data indicator to be the message text.  When the end of textis
     successfully received and stored, the SMTP-receiver sends a "250 OK"
     reply.
1102
     Since the mail data is sent on the transmission channel, the end


of
     mail data must be indicated so that the command and reply dialog can
     be resumed.  SMTP indicates the end of the mail data by sending a
     line containing only a "." (period or full stop).  A transparency
     procedure is used to prevent this from interfering with the user's
     text (see Section 4.5.2).
1109
     The end of mail data indicator also confirms the mail transactionand
     tells the SMTP server to now process the stored recipients and mail
1112
1113
1114
1115

1117











































  Internet-Draft        RFC5321-Numbered               Dec 2008
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1120
     data.  If accepted, the SMTP server returns a "250 OK" reply.  The
     DATA command can fail at only two points in the protocol exchange:
1123
     If there was no MAIL, or no RCPT, command, or all such commands were
     rejected, the server MAY return a "command out of sequence" (503)or
     "no valid recipients" (554) reply in response to the DATA command.
     If one of those replies (or any other 5yz reply) is received, the
     client MUST NOT send the message data; more generally, message data
     MUST NOT be sent unless a 354 reply is received.
1130
     If the verb is initially accepted and the 354 reply issued, the DATA
     command should fail only if the mail transaction was incomplete (for
     example, no recipients), if resources were unavailable (including, of
     course, the server unexpectedly becoming unavailable), or if the
     server determines that the message should be rejected for policy or
     other reasons.
1137
     However, in practice, some servers do not perform recipient
     verification until after the message text is received.  These servers
     SHOULD treat a failure for one or more recipients as a "subsequent
     failure" and return a mail message as discussed in Section 6 and,in
     particular, in Section 6.1.  Using a "550 mailbox not found" (or
     equivalent) reply code after the data are accepted makes it difficult
     or impossible for the client to determine which recipients failed.
1145
     When the RFC 822 format ([28], [4]) is being used, the mail data
     include the header fields such as those named Date, Subject, To, Cc,
     and From.  Server SMTP systems SHOULD NOT reject messages based on
     perceived defects in the RFC 822 or MIME (RFC 2045 [21]) message
     header section or message body.  In particular, they MUST NOT reject
     messages in which the numbers of Resent-header fields do not match or
     Resent-to appears without Resent-from and/or Resent-date.
1153
     Mail transaction commands MUST be used in the order discussed above.
1155


  3.4.  Forwarding for Address Correction or Updating
1157
     Forwarding support is most often required to consolidate and simplify
     addresses within, or relative to, some enterprise and less frequently
     to establish addresses to link a person's prior address with a
     current one.  Silent forwarding of messages (without server
     notification to the sender), for security or non-disclosure purposes,
     is common in the contemporary Internet.
1164
     In both the enterprise and the "new address" cases, information
     hiding (and sometimes security) considerations argue against exposure
     of the "final" address through the SMTP protocol as a side effectof
     the forwarding activity.  This may be especially important when the
1169
1170
1171

1173






































  Internet-Draft        RFC5321-Numbered               Dec 2008
1175
1176
     final address may not even be reachable by the sender.  Consequently,
     the "forwarding" mechanisms described in Section 3.2 of RFC 821, and
     especially the 251 (corrected destination) and 551 reply codes from
     RCPT must be evaluated carefully by implementers and, when they are
     available, by those configuring systems (see also Section 7.4).
1182
     In particular:
1184
     o  Servers MAY forward messages when they are aware of an address
        change.  When they do so, they MAY either provide address-updating
        information with a 251 code, or may forward "silently" and return
        a 250 code.  However, if a 251 code is used, they MUST NOT assume
        that the client will actually update address information or even
        return that information to the user.
1191
     Alternately,
1193
     o  Servers MAY reject messages or return them as non-deliverable when
        they cannot be delivered precisely as addressed.  When they doso,
        they MAY either provide address-updating information with a 551
        code, or may reject the message as undeliverable with a 550 code
        and no address-specific information.  However, if a 551 code is
        used, they MUST NOT assume that the client will actually update
        address information or even return that information to the user.
1201
     SMTP server implementations that support the 251 and/or 551 reply
     codes SHOULD provide configuration mechanisms so that sites that
     conclude that they would undesirably disclose information can disable
     or restrict their use.
1206
  3.5.  Commands for Debugging Addresses
1208
  3.5.1.  Overview
1210
     SMTP provides commands to verify a user name or obtain the content of
     a mailing list.  This is done with the VRFY and EXPN commands, which
     have character string arguments.  Implementations SHOULD support


VRFY
     and EXPN (however, see Section 3.5.2 and Section 7.3).
1215
     For the VRFY command, the string is a user name or a user name and
     domain (see below).  If a normal (i.e., 250) response is returned,
     the response MAY include the full name of the user and MUST include
     the mailbox of the user.  It MUST be in either of the following
     forms:
1221
        User Name <local-part@domain>
        local-part@domain
1224
1225
1226
1227

1229










































  Internet-Draft        RFC5321-Numbered               Dec 2008
1231
1232
     When a name that is the argument to VRFY could identify more thanone
     mailbox, the server MAY either note the ambiguity or identify the
     alternatives.  In other words, any of the following are legitimate
     responses to VRFY:
1237
        553 User ambiguous
1239
     or
1241
        553- Ambiguous; Possibilities are
        553-Joe Smith <jsmith@foo.com>
        553-Harry Smith <hsmith@foo.com>
        553 Melvin Smith <dweep@foo.com>
1246
     or
1248
        553-Ambiguous; Possibilities
        553- <jsmith@foo.com>
        553- <hsmith@foo.com>
        553 <dweep@foo.com>
1253
     Under normal circumstances, a client receiving a 553 reply would be
     expected to expose the result to the user.  Use of exactly the forms
     given, and the "user ambiguous" or "ambiguous" keywords, possibly
     supplemented by extended reply codes, such as those described in RFC
     3463 [25], will facilitate automated translation into other languages
     as needed.  Of course, a client that was highly automated or thatwas
     operating in another language than English might choose to try to
     translate the response to return some other indication to the user
     than the literal text of the reply, or to take some automated action
     such as consulting a directory service for additional information
     before reporting to the user.
1265
     For the EXPN command, the string identifies a mailing list, and the
     successful (i.e., 250) multiline response MAY include the full name
     of the users and MUST give the mailboxes on the mailing list.
1269
     In some hosts, the distinction between a mailing list and an alias
     for a single mailbox is a bit fuzzy, since a common data structure
     may hold both types of entries, and it is possible to have mailing
     lists containing only one mailbox.  If a request is made to apply


     VRFY to a mailing list, a positive response MAY be given if a message
     so addressed would be delivered to everyone on the list, otherwise an
     error SHOULD be reported (e.g., "550 That is a mailing list, not a
     user" or "252 Unable to verify members of mailing list").  If a
     request is made to expand a user name, the server MAY return a
1279
1280
1281
1282
1283

1285















































  Internet-Draft        RFC5321-Numbered               Dec 2008
1287
1288
     positive response consisting of a list containing one name, or an
     error MAY be reported (e.g., "550 That is a user name, not a mailing
     list").
1292
     In the case of a successful multiline reply (normal for EXPN),
     exactly one mailbox is to be specified on each line of the reply.
     The case of an ambiguous request is discussed above.
1296
     "User name" is a fuzzy term and has been used deliberately.  An
     implementation of the VRFY or EXPN commands MUST include at least
     recognition of local mailboxes as "user names".  However, since
     current Internet practice often results in a single host handling
     mail for multiple domains, hosts, especially hosts that provide this
     functionality, SHOULD accept the "local-part@domain" form as a "user
     name"; hosts MAY also choose to recognize other strings as "user
     names".
1305
     The case of expanding a mailbox list requires a multiline reply, such
     as:
1308
        C: EXPN Example-People
        S: 250-Jon Postel <Postel@isi.edu>
        S: 250-Fred Fonebone <Fonebone@physics.foo-u.edu>
        S: 250 Sam Q. Smith <SQSmith@specific.generic.com>
1313
     or
1315
        C: EXPN Executive-Washroom-List
        S: 550 Access Denied to You.
1318
     The character string arguments of the VRFY and EXPN commands cannot
     be further restricted due to the variety of implementations of the
     user name and mailbox list concepts.  On some systems, it may be
     appropriate for the argument of the EXPN command to be a file name
     for a file containing a mailing list, but again there are a variety
     of file naming conventions in the Internet.  Similarly, historical
     variations in what is returned by these commands are such that the
     response SHOULD be interpreted very carefully, if at all, and SHOULD
     generally only be used for diagnostic purposes.
1328
  3.5.2.  VRFY Normal Response
1330
     When normal (2yz or 551) responses are returned from a VRFY or EXPN


     request, the reply MUST include the <Mailbox> name using a
     "<local-part@domain>" construction, where "domain" is a fully-
     qualified domain name.  In circumstances exceptional enough to
     justify violating the intent of this specification, free-form text
     MAY be returned.  In order to facilitate parsing by both computers
1337
1338
1339

1341


















































  Internet-Draft        RFC5321-Numbered               Dec 2008
1343
1344
     and people, addresses SHOULD appear in pointed brackets.  When
     addresses, rather than free-form debugging information, are returned,
     EXPN and VRFY MUST return only valid domain addresses that are usable
     in SMTP RCPT commands.  Consequently, if an address implies delivery
     to a program or other system, the mailbox name used to reach that
     target MUST be given.  Paths (explicit source routes) MUST NOT be
     returned by VRFY or EXPN.
1352
     Server implementations SHOULD support both VRFY and EXPN.  For
     security reasons, implementations MAY provide local installationsa
     way to disable either or both of these commands through configuration
     options or the equivalent (see Section 7.3).  When these commandsare
     supported, they are not required to work across relays when relaying
     is supported.  Since they were both optional in RFC 821, but VRFYwas
     made mandatory in RFC 1123 [3], if EXPN is supported, it MUST be
     listed as a service extension in an EHLO response.  VRFY MAY be
     listed as a convenience but, since support for it is required, SMTP
     clients are not required to check for its presence on the extension
     list before using it.
1364
  3.5.3.  Meaning of VRFY or EXPN Success Response
1366
     A server MUST NOT return a 250 code in response to a VRFY or EXPN
     command unless it has actually verified the address.  In particular,
     a server MUST NOT return 250 if all it has done is to verify thatthe
     syntax given is valid.  In that case, 502 (Command not implemented)
     or 500 (Syntax error, command unrecognized) SHOULD be returned.  As
     stated elsewhere, implementation (in the sense of actually validating
     addresses and returning information) of VRFY and EXPN are strongly
     recommended.  Hence, implementations that return 500 or 502 for VRFY
     are not in full compliance with this specification.
1376
     There may be circumstances where an address appears to be valid but
     cannot reasonably be verified in real time, particularly when a
     server is acting as a mail exchanger for another server or domain.
     "Apparent validity", in this case, would normally involve at leas


t
     syntax checking and might involve verification that any domains
     specified were ones to which the host expected to be able to relay
     mail.  In these situations, reply code 252 SHOULD be returned.  These
     cases parallel the discussion of RCPT verification in Section 2.1.
     Similarly, the discussion in Section 3.4 applies to the use of reply
     codes 251 and 551 with VRFY (and EXPN) to indicate addresses thatare
     recognized but that would be forwarded or rejected were mail received
     for them.  Implementations generally SHOULD be more aggressive about
     address verification in the case of VRFY than in the case of RCPT,
     even if it takes a little longer to do so.
1391
1392
1393
1394
1395

1397




































  Internet-Draft        RFC5321-Numbered               Dec 2008
1399
1400
  3.5.4.  Semantics and Applications of EXPN
1402
     EXPN is often very useful in debugging and understanding problems
     with mailing lists and multiple-target-address aliases.  Some systems
     have attempted to use source expansion of mailing lists as a means of
     eliminating duplicates.  The propagation of aliasing systems with
     mail on the Internet for hosts (typically with MX and CNAME DNS
     records), for mailboxes (various types of local host aliases), and in
     various proxying arrangements has made it nearly impossible for these
     strategies to work consistently, and mail systems SHOULD NOT attempt
     them.
1412
  3.6.  Relaying and Mail Routing
1414
  3.6.1.  Source Routes and Relaying
1416
     In general, the availability of Mail eXchanger records in the domain
     name system (RFC 1035 [2], RFC 974 [12]) makes the use of explicit
     source routes in the Internet mail system unnecessary.  Many
     historical problems with the interpretation of explicit source routes
     have made their use undesirable.  SMTP clients SHOULD NOT generate
     explicit source routes except under unusual circumstances.  SMTP
     servers MAY decline to act as mail relays or to accept addresses that
     specify source routes.  When route information is encountered, SMTP
     servers MAY ignore the route information and simply send to the final
     destination specified as the last element in the route and SHOULDdo
     so.  There has been an invalid practice of using names that do not
     appear in the DNS as destination names, with the senders countingon
     the intermediate hosts specified in source routing to resolve any
     problems.  If source routes are stripped, this practice will cause
     failures.  This is one of several reasons why SMTP clients MUST NOT
     generate invalid source routes or depend on serial resolution of
     names.
1434
     When source routes are not used, the process described in RFC 821for
     constructing a reverse-path from the forward-path is not applicable


     and the reverse-path at the time of delivery will simply be the
     address that appeared in the MAIL command.
1439
  3.6.2.  Mail eXchange Records and Relaying
1441
     A relay SMTP server is usually the target of a DNS MX record that
     designates it, rather than the final delivery system.  The relay
     server may accept or reject the task of relaying the mail in the same
     way it accepts or rejects mail for a local user.  If it accepts the
     task, it then becomes an SMTP client, establishes a transmission
     channel to the next SMTP server specified in the DNS (according to
     the rules in Section 5), and sends it the mail.  If it declines to
1449
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  Internet-Draft        RFC5321-Numbered               Dec 2008
1455
1456
     relay mail to a particular address for policy reasons, a 550 response
     SHOULD be returned.
1459
     This specification does not deal with the verification of return
     paths for use in delivery notifications.  Recent work, such as that
     on SPF [29] and DKIM [30] [31], has been done to provide ways to
     ascertain that an address is valid or belongs to the person who
     actually sent the message.  A server MAY attempt to verify the return
     path before using its address for delivery notifications, but methods
     of doing so are not defined here nor is any particular method
     recommended at this time.
1468
  3.6.3.  Message Submission Servers as Relays
1470
     Many mail-sending clients exist, especially in conjunction with
     facilities that receive mail via POP3 or IMAP, that have limited
     capability to support some of the requirements of this specification,
     such as the ability to queue messages for subsequent delivery
     attempts.  For these clients, it is common practice to make private
     arrangements to send all messages to a single server for processing
     and subsequent distribution.  SMTP, as specified here, is not ideally
     suited for this role.  A standardized mail submission protocol has
     been developed that is gradually superseding practices based on SMTP
     (see RFC 4409 [18]).  In any event, because these arrangements are
     private and fall outside the scope of this specification, they are
     not described here.
1483
     It is important to note that MX records can point to SMTP servers
     that act as gateways into other environments, not just SMTP relays
     and final delivery systems; see Sections 3.7 and 5.
1487
     If an SMTP server has accepted the task of relaying the mail and
     later finds that the destination is incorrect or that the mail cannot
     be delivered for some other reason, then it MUST construct an
     "undeliverable mail" notification message and send it to the
     originator of the undeliverable mail (as indicated by the reverse-
     path).  Formats specified for non-delivery reports by other standards
     (see, for example, RFC 3461 [32] and RFC 3464 [33]) SHOULD be used if


     possible.
1496
     This notification message must be from the SMTP server at the relay
     host or the host that first determines that delivery cannot be
     accomplished.  Of course, SMTP servers MUST NOT send notification
     messages about problems transporting notification messages.  One way
     to prevent loops in error reporting is to specify a null reverse-path
     in the MAIL command of a notification message.  When such a message
     is transmitted, the reverse-path MUST be set to null (see
1504
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1507

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  Internet-Draft        RFC5321-Numbered               Dec 2008
1511
1512
     Section 4.5.5 for additional discussion).  A MAIL command with a null
     reverse-path appears as follows:
1515
        MAIL FROM:<>
1517
     As discussed in Section 6.4, a relay SMTP has no need to inspect or
     act upon the header section or body of the message data and MUST NOT
     do so except to add its own "Received:" header field (Section 4.4)
     and, optionally, to attempt to detect looping in the mail system (see
     Section 6.3).  Of course, this prohibition also applies to any
     modifications of these header fields or text (see also Section 7.9).
1524
  3.7.  Mail Gatewaying
1526
     While the relay function discussed above operates within the Internet
     SMTP transport service environment, MX records or various forms of
     explicit routing may require that an intermediate SMTP server perform
     a translation function between one transport service and another.As
     discussed in Section 2.3.10, when such a system is at the boundary
     between two transport service environments, we refer to it as a
     "gateway" or "gateway SMTP".
1534
     Gatewaying mail between different mail environments, such as
     different mail formats and protocols, is complex and does not easily
     yield to standardization.  However, some general requirements maybe
     given for a gateway between the Internet and another mail
     environment.
1540
  3.7.1.  Header Fields in Gatewaying
1542
     Header fields MAY be rewritten when necessary as messages are
     gatewayed across mail environment boundaries.  This may involve
     inspecting the message body or interpreting the local-part of the
     destination address in spite of the prohibitions in Section 6.4.
1547
     Other mail systems gatewayed to the Internet often use a subset of
     the RFC 822 header section or provide similar functionality with a
     different syntax, but some of these mail systems do not have an
     equivalent to the SMTP envelope.  Therefore, when a message leaves


     the Internet environment, it may be necessary to fold the SMTP
     envelope information into the message header section.  A possible
     solution would be to create new header fields to carry the envelope
     information (e.g., "X-SMTP-MAIL:" and "X-SMTP-RCPT:"); however, this
     would require changes in mail programs in foreign environments and
     might risk disclosure of private information (see Section 7.2).
1558
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  Internet-Draft        RFC5321-Numbered               Dec 2008
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1568
  3.7.2.  Received Lines in Gatewaying
1570
     When forwarding a message into or out of the Internet environment, a
     gateway MUST prepend a Received: line, but it MUST NOT alter in any
     way a Received: line that is already in the header section.
1574
     "Received:" header fields of messages originating from other
     environments may not conform exactly to this specification.  However,
     the most important use of Received: lines is for debugging mail
     faults, and this debugging can be severely hampered by well-meaning
     gateways that try to "fix" a Received: line.  As another consequence
     of trace header fields arising in non-SMTP environments, receiving
     systems MUST NOT reject mail based on the format of a trace header
     field and SHOULD be extremely robust in the light of unexpected
     information or formats in those header fields.
1584
     The gateway SHOULD indicate the environment and protocol in the "via"
     clauses of Received header field(s) that it supplies.
1587
  3.7.3.  Addresses in Gatewaying
1589
     From the Internet side, the gateway SHOULD accept all valid address
     formats in SMTP commands and in the RFC 822 header section, and all
     valid RFC 822 messages.  Addresses and header fields generated by
     gateways MUST conform to applicable standards (including this oneand
     RFC 5322 [4]).  Gateways are, of course, subject to the same rules
     for handling source routes as those described for other SMTP systems
     in Section 3.3.
1597
  3.7.4.  Other Header Fields in Gatewaying
1599
     The gateway MUST ensure that all header fields of a message that it
     forwards into the Internet mail environment meet the requirementsfor
     Internet mail.  In particular, all addresses in "From:", "To:",
     "Cc:", etc., header fields MUST be transformed (if necessary) to
     satisfy the standard header syntax of RFC 5322 [4], MUST reference
     only fully-qualified domain names, and MUST be effective and useful
     for sending replies.  The translation algorithm used to convert m


ail
     from the Internet protocols to another environment's protocol SHOULD
     ensure that error messages from the foreign mail environment are
     delivered to the reverse-path from the SMTP envelope, not to an
     address in the "From:", "Sender:", or similar header fields of the
     message.
1612
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1614
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  Internet-Draft        RFC5321-Numbered               Dec 2008
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1624
  3.7.5.  Envelopes in Gatewaying
1626
     Similarly, when forwarding a message from another environment into
     the Internet, the gateway SHOULD set the envelope return path in
     accordance with an error message return address, if supplied by the
     foreign environment.  If the foreign environment has no equivalent
     concept, the gateway must select and use a best approximation, with
     the message originator's address as the default of last resort.
1633
  3.8.  Terminating Sessions and Connections
1635
     An SMTP connection is terminated when the client sends a QUIT
     command.  The server responds with a positive reply code, after which
     it closes the connection.
1639
     An SMTP server MUST NOT intentionally close the connection under
     normal operational circumstances (see Section 7.8) except:
1642
     o  After receiving a QUIT command and responding with a 221 reply.
1644
     o  After detecting the need to shut down the SMTP service and
        returning a 421 response code.  This response code can be issued
        after the server receives any command or, if necessary,
        asynchronously from command receipt (on the assumption that the
        client will receive it after the next command is issued).
1650
     o  After a timeout, as specified in Section 4.5.3.2, occurs waiting
        for the client to send a command or data.
1653
     In particular, a server that closes connections in response to
     commands that are not understood is in violation of this
     specification.  Servers are expected to be tolerant of unknown
     commands, issuing a 500 reply and awaiting further instructions from
     the client.
1659
     An SMTP server that is forcibly shut down via external means SHOULD
     attempt to send a line containing a 421 response code to the SMTP
     client before exiting.  The SMTP client will normally read the 421
     response code after sending its next command.
1664
     SMTP clients that experience a connection close, reset, or other
     communications failure due to circumstances not under their control


     (in violation of the intent of this specification but sometimes
     unavoidable) SHOULD, to maintain the robustness of the mail system,
     treat the mail transaction as if a 451 response had been receivedand
     act accordingly.
1671
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  Internet-Draft        RFC5321-Numbered               Dec 2008
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1680
  3.9.  Mailing Lists and Aliases
1682
     An SMTP-capable host SHOULD support both the alias and the list
     models of address expansion for multiple delivery.  When a message is
     delivered or forwarded to each address of an expanded list form, the
     return address in the envelope ("MAIL FROM:") MUST be changed to be
     the address of a person or other entity who administers the list.
     However, in this case, the message header section (RFC 5322 [4]) MUST
     be left unchanged; in particular, the "From" field of the header
     section is unaffected.
1691
     An important mail facility is a mechanism for multi-destination
     delivery of a single message, by transforming (or "expanding" or
     "exploding") a pseudo-mailbox address into a list of destination
     mailbox addresses.  When a message is sent to such a pseudo-mailbox
     (sometimes called an "exploder"), copies are forwarded or
     redistributed to each mailbox in the expanded list.  Servers SHOULD
     simply utilize the addresses on the list; application of heuristics
     or other matching rules to eliminate some addresses, such as thatof
     the originator, is strongly discouraged.  We classify such a pseudo-
     mailbox as an "alias" or a "list", depending upon the expansion
     rules.
1703
  3.9.1.  Alias
1705
     To expand an alias, the recipient mailer simply replaces the pseudo-
     mailbox address in the envelope with each of the expanded addresses
     in turn; the rest of the envelope and the message body are left
     unchanged.  The message is then delivered or forwarded to each
     expanded address.
1711
  3.9.2.  List
1713
     A mailing list may be said to operate by "redistribution" rather than
     by "forwarding".  To expand a list, the recipient mailer replacesthe
     pseudo-mailbox address in the envelope with each of the expanded
     addresses in turn.  The return (backward-pointing) address in the
     envelope is changed so that all error messages generated by the final
     deliveries will be returned to a list administrator, not to the
     message originator, who generally has no control over the contents of


     the list and will typically find error messages annoying.  Note that
     the key difference between handling aliases (Section 3.9.1) and
     forwarding (this subsection) is the change to the backward-pointing
     address in this case.  When a list constrains its processing to the
     very limited set of modifications and actions described here, it is
     attempting to emulate an MTA; such lists can be treated as a
     continuation in email transit.
1728
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  Internet-Draft        RFC5321-Numbered               Dec 2008
1735
1736
     There exist mailing lists that perform additional, sometimes
     extensive, modifications to a message and its envelope.  Such mailing
     lists need to be viewed as full MUAs, which accept a delivery and
     post a new message.
1741
  4.  The SMTP Specifications
1743
  4.1.  SMTP Commands
1745
  4.1.1.  Command Semantics and Syntax
1747
     The SMTP commands define the mail transfer or the mail system
     function requested by the user.  SMTP commands are character strings
     terminated by <CRLF>.  The commands themselves are alphabetic
     characters terminated by <SP> if parameters follow and <CRLF>
     otherwise.  (In the interest of improved interoperability, SMTP
     receivers SHOULD tolerate trailing white space before the terminating
     <CRLF>.)  The syntax of the local part of a mailbox MUST conform to
     receiver site conventions and the syntax specified in Section 4.1.2.
     The SMTP commands are discussed below.  The SMTP replies are
     discussed in Section 4.2.
1758
     A mail transaction involves several data objects that are
     communicated as arguments to different commands.  The reverse-path is
     the argument of the MAIL command, the forward-path is the argument of
     the RCPT command, and the mail data is the argument of the DATA
     command.  These arguments or data objects must be transmitted and
     held, pending the confirmation communicated by the end of mail data
     indication that finalizes the transaction.  The model for this is
     that distinct buffers are provided to hold the types of data objects;
     that is, there is a reverse-path buffer, a forward-path buffer, and a
     mail data buffer.  Specific commands cause information to be appended
     to a specific buffer, or cause one or more buffers to be cleared.
1770
     Several commands (RSET, DATA, QUIT) are specified as not permitting
     parameters.  In the absence of specific extensions offered by the
     server and accepted by the client, clients MUST NOT send such
     parameters and servers SHOULD reject commands containing them as
     having invalid syntax.
1776
  4.1.1.1.  Extended HELLO (EHLO) or HELLO (HELO)
1778
     These commands are used to identify the SMTP client to the SMTP


     server.  The argument clause contains the fully-qualified domain name
     of the SMTP client, if one is available.  In situations in which the
     SMTP client system does not have a meaningful domain name (e.g., when
     its address is dynamically allocated and no reverse mapping record is
1784
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  Internet-Draft        RFC5321-Numbered               Dec 2008
1791
1792
     available), the client SHOULD send an address literal (see
     Section 4.1.3).
1795
     RFC 2821, and some earlier informal practices, encouraged following
     the literal by information that would help to identify the client
     system.  That convention was not widely supported, and many SMTP
     servers considered it an error.  In the interest of interoperability,
     it is probably wise for servers to be prepared for this string to
     occur, but SMTP clients SHOULD NOT send it.
1802
     The SMTP server identifies itself to the SMTP client in the
     connection greeting reply and in the response to this command.
1805
     A client SMTP SHOULD start an SMTP session by issuing the EHLO
     command.  If the SMTP server supports the SMTP service extensions, it
     will give a successful response, a failure response, or an error
     response.  If the SMTP server, in violation of this specification,
     does not support any SMTP service extensions, it will generate an
     error response.  Older client SMTP systems MAY, as discussed above,
     use HELO (as specified in RFC 821) instead of EHLO, and servers MUST
     support the HELO command and reply properly to it.  In any event,a
     client MUST issue HELO or EHLO before starting a mail transaction.
1815
     These commands, and a "250 OK" reply to one of them, confirm that
     both the SMTP client and the SMTP server are in the initial state,
     that is, there is no transaction in progress and all state tablesand
     buffers are cleared.
1820
     Syntax:
1822
     ehlo           = "EHLO" SP ( Domain / address-literal ) CRLF
1824
     helo           = "HELO" SP Domain CRLF
1826
     Normally, the response to EHLO will be a multiline reply.  Each line
     of the response contains a keyword and, optionally, one or more
     parameters.  Following the normal syntax for multiline replies, these
     keywords follow the code (250) and a hyphen for all but the last
     line, and the code and a space for the last line.  The syntax fora
     positive response, using the ABNF notation and terminal symbols of
     RFC 5234 [7], is:


1834
     ehlo-ok-rsp    = ( "250" SP Domain [ SP ehlo-greet ] CRLF )
                      / ( "250-" Domain [ SP ehlo-greet ] CRLF
                      *( "250-" ehlo-line CRLF )
                      "250" SP ehlo-line CRLF )
1839
1840
1841
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1843

1845


















































  Internet-Draft        RFC5321-Numbered               Dec 2008
1847
1848
     ehlo-greet     = 1*(%d0-9 / %d11-12 / %d14-127)
                      ; string of any characters other than CR or LF
1851
     ehlo-line      = ehlo-keyword *( SP ehlo-param )
1853
     ehlo-keyword   = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
                      ; additional syntax of ehlo-params depends on
                      ; ehlo-keyword
1857
     ehlo-param     = 1*(%d33-126)
                      ; any CHAR excluding <SP> and all
                      ; control characters (US-ASCII 0-31 and 127
                      ; inclusive)
1862
     Although EHLO keywords may be specified in upper, lower, or mixed
     case, they MUST always be recognized and processed in a case-
     insensitive manner.  This is simply an extension of practices
     specified in RFC 821 and Section 2.4.
1867
     The EHLO response MUST contain keywords (and associated parameters if
     required) for all commands not listed as "required" in Section 4.5.1
     excepting only private-use commands as described in Section 4.1.5.
     Private-use commands MAY be listed.
1872
  4.1.1.2.  MAIL (MAIL)
1874
     This command is used to initiate a mail transaction in which the mail
     data is delivered to an SMTP server that may, in turn, deliver itto
     one or more mailboxes or pass it on to another system (possibly using
     SMTP).  The argument clause contains a reverse-path and may contain
     optional parameters.  In general, the MAIL command may be sent only
     when no mail transaction is in progress, see Section 4.1.4.
1881
     The reverse-path consists of the sender mailbox.  Historically, that
     mailbox might optionally have been preceded by a list of hosts, but
     that behavior is now deprecated (see Appendix C).  In some types of
     reporting messages for which a reply is likely to cause a mail loop
     (for example, mail delivery and non-delivery notifications), the
     reverse-path may be null (see Section 3.6).
1888
     This command clears the reverse-path buffer, the forward-path buffer,
     and the mail data buffer, and it inserts the reverse-path informa


tion
     from its argument clause into the reverse-path buffer.
1892
     If service extensions were negotiated, the MAIL command may also
     carry parameters associated with a particular service extension.
1895
1896
1897
1898
1899

1901


















































  Internet-Draft        RFC5321-Numbered               Dec 2008
1903
1904
     Syntax:
1906
     mail = "MAIL FROM:" Reverse-path
                                         [SP Mail-parameters] CRLF
1909
  4.1.1.3.  RECIPIENT (RCPT)
1911
     This command is used to identify an individual recipient of the mail
     data; multiple recipients are specified by multiple uses of this
     command.  The argument clause contains a forward-path and may contain
     optional parameters.
1916
     The forward-path normally consists of the required destination
     mailbox.  Sending systems SHOULD NOT generate the optional list of
     hosts known as a source route.  Receiving systems MUST recognize
     source route syntax but SHOULD strip off the source route
     specification and utilize the domain name associated with the mailbox
     as if the source route had not been provided.
1923
     Similarly, relay hosts SHOULD strip or ignore source routes, and
     names MUST NOT be copied into the reverse-path.  When mail reaches
     its ultimate destination (the forward-path contains only a
     destination mailbox), the SMTP server inserts it into the destination
     mailbox in accordance with its host mail conventions.
1929
     This command appends its forward-path argument to the forward-path
     buffer; it does not change the reverse-path buffer nor the mail data
     buffer.
1933
     For example, mail received at relay host xyz.com with envelope
     commands
1936
        MAIL FROM:<userx@y.foo.org>
        RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org>
1939
     will normally be sent directly on to host d.bar.org with envelope
     commands
1942
        MAIL FROM:<userx@y.foo.org>
        RCPT TO:<userc@d.bar.org>
1945
     As provided in Appendix C, xyz.com MAY also choose to relay the
     message to hosta.int, using the envelope commands
1948
        MAIL FROM:<userx@y.foo.org>
        RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org>
1951


1952
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  Internet-Draft        RFC5321-Numbered               Dec 2008
1959
1960
     or to jkl.org, using the envelope commands
1962
        MAIL FROM:<userx@y.foo.org>
        RCPT TO:<@jkl.org:userc@d.bar.org>
1965
     Attempting to use relaying this way is now strongly discouraged.
     Since hosts are not required to relay mail at all, xyz.com MAY also
     reject the message entirely when the RCPT command is received, using
     a 550 code (since this is a "policy reason").
1970
     If service extensions were negotiated, the RCPT command may also
     carry parameters associated with a particular service extension
     offered by the server.  The client MUST NOT transmit parameters other
     than those associated with a service extension offered by the server
     in its EHLO response.
1976
     Syntax:
1978
        rcpt = "RCPT TO:" ( "<Postmaster@" Domain ">" / "<Postmaster>"/
                    Forward-path ) [SP Rcpt-parameters] CRLF
1981
                    Note that, in a departure from the usual rules for
                    local-parts, the "Postmaster" string shown above is
                    treated as case-insensitive.
1985
  4.1.1.4.  DATA (DATA)
1987
     The receiver normally sends a 354 response to DATA, and then treats
     the lines (strings ending in <CRLF> sequences, as described in
     Section 2.3.7) following the command as mail data from the sender.
     This command causes the mail data to be appended to the mail data
     buffer.  The mail data may contain any of the 128 ASCII character
     codes, although experience has indicated that use of control
     characters other than SP, HT, CR, and LF may cause problems and
     SHOULD be avoided when possible.
1996
     The mail data are terminated by a line containing only a period, that
     is, the character sequence "<CRLF>.<CRLF>", where the first <CRLF> is
     actually the terminator of the previous line (see Section 4.5.2).
     This is the end of mail data indication.  The first <CRLF> of this
     terminating sequence is also the <CRLF> that ends the final line of
     the data (message text) or, if there was no mail data, ends the DATA


     command itself (the "no mail data" case does not conform to this
     specification since it would require that neither the trace header
     fields required by this specification nor the message header section
     required by RFC 5322 [4] be transmitted).  An extra <CRLF> MUST NOT
     be added, as that would cause an empty line to be added to the
     message.  The only exception to this rule would arise if the message
2009
2010
2011

2013















































  Internet-Draft        RFC5321-Numbered               Dec 2008
2015
2016
     body were passed to the originating SMTP-sender with a final "line"
     that did not end in <CRLF>; in that case, the originating SMTP system
     MUST either reject the message as invalid or add <CRLF> in order to
     have the receiving SMTP server recognize the "end of data" condition.
2021
     The custom of accepting lines ending only in <LF>, as a concession to
     non-conforming behavior on the part of some UNIX systems, has proven
     to cause more interoperability problems than it solves, and SMTP
     server systems MUST NOT do this, even in the name of improved
     robustness.  In particular, the sequence "<LF>.<LF>" (bare line
     feeds, without carriage returns) MUST NOT be treated as equivalent to
     <CRLF>.<CRLF> as the end of mail data indication.
2029
     Receipt of the end of mail data indication requires the server to
     process the stored mail transaction information.  This processing
     consumes the information in the reverse-path buffer, the forward-path
     buffer, and the mail data buffer, and on the completion of this
     command these buffers are cleared.  If the processing is successful,
     the receiver MUST send an OK reply.  If the processing fails, the
     receiver MUST send a failure reply.  The SMTP model does not allow
     for partial failures at this point: either the message is accepted by
     the server for delivery and a positive response is returned or itis
     not accepted and a failure reply is returned.  In sending a positive
     "250 OK" completion reply to the end of data indication, the receiver
     takes full responsibility for the message (see Section 6.1).  Errors
     that are diagnosed subsequently MUST be reported in a mail message,
     as discussed in Section 4.4.
2044
     When the SMTP server accepts a message either for relaying or for
     final delivery, it inserts a trace record (also referred to
     interchangeably as a "time stamp line" or "Received" line) at thetop
     of the mail data.  This trace record indicates the identity of the
     host that sent the message, the identity of the host that received
     the message (and is inserting this time stamp), and the date and time
     the message was received.  Relayed messages will have multiple ti


me
     stamp lines.  Details for formation of these lines, including their
     syntax, is specified in Section 4.4.
2054
     Additional discussion about the operation of the DATA command appears
     in Section 3.3.
2057
     Syntax:
2059
        data = "DATA" CRLF
2061
2062
2063
2064
2065
2066
2067

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  Internet-Draft        RFC5321-Numbered               Dec 2008
2071
2072
  4.1.1.5.  RESET (RSET)
2074
     This command specifies that the current mail transaction will be
     aborted.  Any stored sender, recipients, and mail data MUST be
     discarded, and all buffers and state tables cleared.  The receiver
     MUST send a "250 OK" reply to a RSET command with no arguments.  A
     reset command may be issued by the client at any time.  It is
     effectively equivalent to a NOOP (i.e., it has no effect) if issued
     immediately after EHLO, before EHLO is issued in the session, after
     an end of data indicator has been sent and acknowledged, or
     immediately before a QUIT.  An SMTP server MUST NOT close the
     connection as the result of receiving a RSET; that action is reserved
     for QUIT (see Section 4.1.1.10).
2086
     Since EHLO implies some additional processing and response by the
     server, RSET will normally be more efficient than reissuing that
     command, even though the formal semantics are the same.
2090
     There are circumstances, contrary to the intent of this
     specification, in which an SMTP server may receive an indication that
     the underlying TCP connection has been closed or reset.  To preserve
     the robustness of the mail system, SMTP servers SHOULD be prepared
     for this condition and SHOULD treat it as if a QUIT had been received
     before the connection disappeared.
2097
     Syntax:
2099
        rset = "RSET" CRLF
2101
  4.1.1.6.  VERIFY (VRFY)
2103
     This command asks the receiver to confirm that the argument
     identifies a user or mailbox.  If it is a user name, information is
     returned as specified in Section 3.5.
2107
     This command has no effect on the reverse-path buffer, the forward-
     path buffer, or the mail data buffer.
2110
     Syntax:
2112
        vrfy = "VRFY" SP String CRLF
2114
2115
2116


2117
2118
2119
2120
2121
2122
2123

2125





















































  Internet-Draft        RFC5321-Numbered               Dec 2008
2127
2128
  4.1.1.7.  EXPAND (EXPN)
2130
     This command asks the receiver to confirm that the argument
     identifies a mailing list, and if so, to return the membership of
     that list.  If the command is successful, a reply is returned
     containing information as described in Section 3.5.  This reply will
     have multiple lines except in the trivial case of a one-member list.
2136
     This command has no effect on the reverse-path buffer, the forward-
     path buffer, or the mail data buffer, and it may be issued at any
     time.
2140
     Syntax:
2142
        expn = "EXPN" SP String CRLF
2144
  4.1.1.8.  HELP (HELP)
2146
     This command causes the server to send helpful information to the
     client.  The command MAY take an argument (e.g., any command name)
     and return more specific information as a response.
2150
     This command has no effect on the reverse-path buffer, the forward-
     path buffer, or the mail data buffer, and it may be issued at any
     time.
2154
     SMTP servers SHOULD support HELP without arguments and MAY support it
     with arguments.
2157
     Syntax:
2159
        help = "HELP" [ SP String ] CRLF
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177


2178
2179

2181


























































  Internet-Draft        RFC5321-Numbered               Dec 2008
2183
2184
  4.1.1.9.  NOOP (NOOP)
2186
     This command does not affect any parameters or previously entered
     commands.  It specifies no action other than that the receiver send a
     "250 OK" reply.
2190
     This command has no effect on the reverse-path buffer, the forward-
     path buffer, or the mail data buffer, and it may be issued at any
     time.  If a parameter string is specified, servers SHOULD ignore it.
2194
     Syntax:
2196
        noop = "NOOP" [ SP String ] CRLF
2198
  4.1.1.10.  QUIT (QUIT)
2200
     This command specifies that the receiver MUST send a "221 OK" reply,
     and then close the transmission channel.
2203
     The receiver MUST NOT intentionally close the transmission channel
     until it receives and replies to a QUIT command (even if there was an
     error).  The sender MUST NOT intentionally close the transmission
     channel until it sends a QUIT command, and it SHOULD wait until it
     receives the reply (even if there was an error response to a previous
     command).  If the connection is closed prematurely due to violations
     of the above or system or network failure, the server MUST cancelany
     pending transaction, but not undo any previously completed
     transaction, and generally MUST act as if the command or transaction
     in progress had received a temporary error (i.e., a 4yz response).
2214
     The QUIT command may be issued at any time.  Any current uncompleted
     mail transaction will be aborted.
2217
     Syntax:
2219
        quit = "QUIT" CRLF
2221
  4.1.1.11.  Mail-Parameter and Rcpt-Parameter Error Responses
2223
     If the server SMTP does not recognize or cannot implement one or more
     of the parameters associated with a particular MAIL FROM or RCPT


TO
     command, it will return code 555.
2227
     If, for some reason, the server is temporarily unable to accommodate
     one or more of the parameters associated with a MAIL FROM or RCPTTO
     command, and if the definition of the specific parameter does not
     mandate the use of another code, it should return code 455.
2232
2233
2234
2235

2237















































  Internet-Draft        RFC5321-Numbered               Dec 2008
2239
2240
     Errors specific to particular parameters and their values will be
     specified in the parameter's defining RFC.
2243
  4.1.2.  Command Argument Syntax
2245
     The syntax of the argument clauses of the above commands (using the
     syntax specified in RFC 5234 [7] where applicable) is given below.
     Some of the productions given below are used only in conjunction with
     source routes as described in Appendix C.  Terminals not defined in
     this document, such as ALPHA, DIGIT, SP, CR, LF, CRLF, are as defined
     in the "core" syntax in Section 6 of RFC 5234 [7] or in the message
     format syntax in RFC 5322 [4].
2253
     Reverse-path   = Path / "<>"
2255
     Forward-path   = Path
2257
     Path           = "<" [ A-d-l ":" ] Mailbox ">"
2259
     A-d-l          = At-domain *( "," At-domain )
                    ; Note that this form, the so-called "source
                    ; route", MUST BE accepted, SHOULD NOT be
                    ; generated, and SHOULD be ignored.
2264
     At-domain      = "@" Domain
2266
     Mail-parameters  = esmtp-param *(SP esmtp-param)
2268
     Rcpt-parameters  = esmtp-param *(SP esmtp-param)
2270
     esmtp-param    = esmtp-keyword ["=" esmtp-value]
2272
     esmtp-keyword  = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
2274
     esmtp-value    = 1*(%d33-60 / %d62-126)
                    ; any CHAR excluding "=", SP, and control
                    ; characters.  If this string is an email address,
                    ; i.e., a Mailbox, then the "xtext" syntax [32]
                    ; SHOULD be used.
2280
     Keyword        = Ldh-str
2282
     Argument       = Atom
2284
     Domain         = sub-domain *("." sub-domain)
2286
2287
2288
2289


2290
2291

2293


























































  Internet-Draft        RFC5321-Numbered               Dec 2008
2295
2296
     sub-domain     = Let-dig [Ldh-str]
2298
     Let-dig        = ALPHA / DIGIT
2300
     Ldh-str        = *( ALPHA / DIGIT / "-" ) Let-dig
2302
     address-literal  = "[" ( IPv4-address-literal /
                      IPv6-address-literal /
                      General-address-literal ) "]"
                      ; See Section 4.1.3
2307
     Mailbox        = Local-part "@" ( Domain / address-literal )
2309
     Local-part     = Dot-string / Quoted-string
                    ; MAY be case-sensitive
2312
2313
     Dot-string     = Atom *("."  Atom)
2315
     Atom           = 1*atext
2317
     Quoted-string  = DQUOTE *QcontentSMTP DQUOTE
2319
     QcontentSMTP   = qtextSMTP / quoted-pairSMTP
2321
     quoted-pairSMTP  = %d92 %d32-126
                      ; i.e., backslash followed by any ASCII
                      ; graphic (including itself) or SPace
2325
     qtextSMTP      = %d32-33 / %d35-91 / %d93-126
                    ; i.e., within a quoted string, any
                    ; ASCII graphic or space is permitted
                    ; without blackslash-quoting except
                    ; double-quote and the backslash itself.
2331
     String         = Atom / Quoted-string
2333
     While the above definition for Local-part is relatively permissive,
     for maximum interoperability, a host that expects to receive mail
     SHOULD avoid defining mailboxes where the Local-part requires (or
     uses) the Quoted-string form or where the Local-part is case-
     sensitive.  For any purposes that require generating or comparing
     Local-parts (e.g., to specific mailbox names), all quoted forms MUST
     be treated as equivalent, and the sending system SHOULD transmit the
     form that uses the minimum quoting possible.
2342
     Systems MUST NOT define mailboxes in such a way as to require theuse
     in SMTP of non-ASCII characters (octets with the high order bit set
2345
2346


2347

2349



























































  Internet-Draft        RFC5321-Numbered               Dec 2008
2351
2352
     to one) or ASCII "control characters" (decimal value 0-31 and 127).
     These characters MUST NOT be used in MAIL or RCPT commands or other
     commands that require mailbox names.
2356
     Note that the backslash, "\", is a quote character, which is usedto
     indicate that the next character is to be used literally (insteadof
     its normal interpretation).  For example, "Joe\,Smith" indicates a
     single nine-character user name string with the comma being the
     fourth character of that string.
2362
     To promote interoperability and consistent with long-standing
     guidance about conservative use of the DNS in naming and applications
     (e.g., see Section 2.3.1 of the base DNS document, RFC 1035 [2]),
     characters outside the set of alphabetic characters, digits, and
     hyphen MUST NOT appear in domain name labels for SMTP clients or
     servers.  In particular, the underscore character is not permitted.
     SMTP servers that receive a command in which invalid character codes
     have been employed, and for which there are no other reasons for
     rejection, MUST reject that command with a 501 response (this rule,
     like others, could be overridden by appropriate SMTP extensions).
2373
  4.1.3.  Address Literals
2375
     Sometimes a host is not known to the domain name system and
     communication (and, in particular, communication to report and repair
     the error) is blocked.  To bypass this barrier, a special literal
     form of the address is allowed as an alternative to a domain name.
     For IPv4 addresses, this form uses four small decimal integers
     separated by dots and enclosed by brackets such as [123.255.37.2],
     which indicates an (IPv4) Internet Address in sequence-of-octets
     form.  For IPv6 and other forms of addressing that might eventually
     be standardized, the form consists of a standardized "tag" that
     identifies the address syntax, a colon, and the address itself, in a
     format specified as part of the relevant standards (i.e., RFC 4291
     [8] for IPv6).
2388
     Specifically:
2390
     IPv4-address-literal  = Snum 3("."  Snum)
2392


     IPv6-address-literal  = "IPv6:" IPv6-addr
2394
     General-address-literal  = Standardized-tag ":" 1*dcontent
2396
     Standardized-tag  = Ldh-str
                       ; Standardized-tag MUST be specified in a
                       ; Standards-Track RFC and registered with IANA
2400
2401
2402
2403

2405

















































  Internet-Draft        RFC5321-Numbered               Dec 2008
2407
2408
     dcontent       = %d33-90 / ; Printable US-ASCII
                    %d94-126 ; excl. "[", "\", "]"
2411
     Snum           = 1*3DIGIT
                    ; representing a decimal integer
                    ; value in the range 0 through 255
2415
     IPv6-addr      = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp
2417
     IPv6-hex       = 1*4HEXDIG
2419
     IPv6-full      = IPv6-hex 7(":" IPv6-hex)
2421
     IPv6-comp      = [IPv6-hex *5(":" IPv6-hex)] "::"
                    [IPv6-hex *5(":" IPv6-hex)]
                    ; The "::" represents at least 2 16-bit groups of
                    ; zeros.  No more than 6 groups in addition to the
                    ; "::" may be present.
2427
     IPv6v4-full    = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal
2429
     IPv6v4-comp    = [IPv6-hex *3(":" IPv6-hex)] "::"
                    [IPv6-hex *3(":" IPv6-hex) ":"]
                    IPv4-address-literal
                    ; The "::" represents at least 2 16-bit groups of
                    ; zeros.  No more than 4 groups in addition to the
                    ; "::" and IPv4-address-literal may be present.
2436
  4.1.4.  Order of Commands
2438
     There are restrictions on the order in which these commands may be
     used.
2441
     A session that will contain mail transactions MUST first be
     initialized by the use of the EHLO command.  An SMTP server SHOULD
     accept commands for non-mail transactions (e.g., VRFY or EXPN)
     without this initialization.
2446
     An EHLO command MAY be issued by a client later in the session.  If
     it is issued after the session begins and the EHLO command is
     acceptable to the SMTP server, the SMTP server MUST clear all buffers
     and reset the state exactly as if a RSET command had been issued.In
     other words, the sequence of RSET followed immediately by EHLO is
     redundant, but not harmful other than in the performance cost of
     executing unnecessary commands.
2454
     If the EHLO command is not acceptable to the SMTP server, 501, 500,


     502, or 550 failure replies MUST be returned as appropriate.  The
2457
2458
2459

2461
























































  Internet-Draft        RFC5321-Numbered               Dec 2008
2463
2464
     SMTP server MUST stay in the same state after transmitting these
     replies that it was in before the EHLO was received.
2467
     The SMTP client MUST, if possible, ensure that the domain parameter
     to the EHLO command is a primary host name as specified for this
     command in Section 2.3.5.  If this is not possible (e.g., when the
     client's address is dynamically assigned and the client does not have
     an obvious name), an address literal SHOULD be substituted for the
     domain name.
2474
     An SMTP server MAY verify that the domain name argument in the EHLO
     command actually corresponds to the IP address of the client.
     However, if the verification fails, the server MUST NOT refuse to
     accept a message on that basis.  Information captured in the
     verification attempt is for logging and tracing purposes.  Note that
     this prohibition applies to the matching of the parameter to its IP
     address only; see Section 7.9 for a more extensive discussion of
     rejecting incoming connections or mail messages.
2483
     The NOOP, HELP, EXPN, VRFY, and RSET commands can be used at any time
     during a session, or without previously initializing a session.  SMTP
     servers SHOULD process these normally (that is, not return a 503
     code) even if no EHLO command has yet been received; clients SHOULD
     open a session with EHLO before sending these commands.
2489
     If these rules are followed, the example in RFC 821 that shows "550
     access denied to you" in response to an EXPN command is incorrect
     unless an EHLO command precedes the EXPN or the denial of access is
     based on the client's IP address or other authentication or
     authorization-determining mechanisms.
2495
     The MAIL command (or the obsolete SEND, SOML, or SAML commands)
     begins a mail transaction.  Once started, a mail transaction consists
     of a transaction beginning command, one or more RCPT commands, and a
     DATA command, in that order.  A mail transaction may be aborted by
     the RSET, a new EHLO, or the QUIT command.  There may be zero or more
     transactions in a session.  MAIL (or SEND, SOML, or SAML) MUST NOT be
     sent if a mail transaction is already open, i.e., it should be se


nt
     only if no mail transaction had been started in the session, or if
     the previous one successfully concluded with a successful DATA
     command, or if the previous one was aborted, e.g., with a RSET ornew
     EHLO.
2507
     If the transaction beginning command argument is not acceptable, a
     501 failure reply MUST be returned and the SMTP server MUST stay in
     the same state.  If the commands in a transaction are out of order to
     the degree that they cannot be processed by the server, a 503 failure
2512
2513
2514
2515

2517








































  Internet-Draft        RFC5321-Numbered               Dec 2008
2519
2520
     reply MUST be returned and the SMTP server MUST stay in the same
     state.
2523
     The last command in a session MUST be the QUIT command.  The QUIT
     command SHOULD be used by the client SMTP to request connection
     closure, even when no session opening command was sent and accepted.
2527
  4.1.5.  Private-Use Commands
2529
     As specified in Section 2.2.2, commands starting in "X" may be used
     by bilateral agreement between the client (sending) and server
     (receiving) SMTP agents.  An SMTP server that does not recognize such
     a command is expected to reply with "500 Command not recognized".An
     extended SMTP server MAY list the feature names associated with these
     private commands in the response to the EHLO command.
2536
     Commands sent or accepted by SMTP systems that do not start with "X"
     MUST conform to the requirements of Section 2.2.2.
2539
  4.2.  SMTP Replies
2541
     Replies to SMTP commands serve to ensure the synchronization of
     requests and actions in the process of mail transfer and to guarantee
     that the SMTP client always knows the state of the SMTP server.
     Every command MUST generate exactly one reply.
2546
     The details of the command-reply sequence are described in
     Section 4.3.
2549
     An SMTP reply consists of a three digit number (transmitted as three
     numeric characters) followed by some text unless specified otherwise
     in this document.  The number is for use by automata to determine
     what state to enter next; the text is for the human user.  The three
     digits contain enough encoded information that the SMTP client need
     not examine the text and may either discard it or pass it on to the
     user, as appropriate.  Exceptions are as noted elsewhere in this
     document.  In particular, the 220, 221, 251, 421, and 551 reply codes
     are associated with message text that must be parsed and interpreted
     by machines.  In the general case, the text may be receiver dependent
     and context dependent, so there are likely to be varying texts fo


r
     each reply code.  A discussion of the theory of reply codes is given
     in Section 4.2.1.  Formally, a reply is defined to be the sequence: a
     three-digit code, <SP>, one line of text, and <CRLF>, or a multiline
     reply (as defined in the same section).  Since, in violation of this
     specification, the text is sometimes not sent, clients that do not
     receive it SHOULD be prepared to process the code alone (with or
     without a trailing space character).  Only the EHLO, EXPN, and HELP
     commands are expected to result in multiline replies in normal
2569
2570
2571

2573










































  Internet-Draft        RFC5321-Numbered               Dec 2008
2575
2576
     circumstances; however, multiline replies are allowed for any
     command.
2579
     In ABNF, server responses are:
2581
     Greeting       = ( "220 " (Domain / address-literal)
                    [ SP textstring ] CRLF ) /
                    ( "220-" (Domain / address-literal)
                    [ SP textstring ] CRLF
                    *( "220-" [ textstring ] CRLF )
                    "220" [ SP textstring ] CRLF )
2588
     textstring     = 1*(%d09 / %d32-126) ; HT, SP, Printable US-ASCII
2590
     Reply-line     = *( Reply-code "-" [ textstring ] CRLF )
                    Reply-code [ SP textstring ] CRLF
2593
     Reply-code     = %x32-35 %x30-35 %x30-39
2595
     where "Greeting" appears only in the 220 response that announces that
     the server is opening its part of the connection.  (Other possible
     server responses upon connection follow the syntax of Reply-line.)
2599
     An SMTP server SHOULD send only the reply codes listed in this
     document.  An SMTP server SHOULD use the text shown in the examples
     whenever appropriate.
2603
     An SMTP client MUST determine its actions only by the reply code,not
     by the text (except for the "change of address" 251 and 551 and, if
     necessary, 220, 221, and 421 replies); in the general case, any text,
     including no text at all (although senders SHOULD NOT send bare
     codes), MUST be acceptable.  The space (blank) following the reply
     code is considered part of the text.  Whenever possible, a receiver-
     SMTP SHOULD test the first digit (severity indication) of the reply
     code.
2612
     The list of codes that appears below MUST NOT be construed as
     permanent.  While the addition of new codes should be a rare and
     significant activity, with supplemental information in the textual
     part of the response being preferred, new codes may be added as the
     result of new Standards or Standards-Track specifications.
     Consequently, a sender-SMTP MUST be prepared to handle codes not
     specified in this document and MUST do so by interpreting the fir


st
     digit only.
2621
     In the absence of extensions negotiated with the client, SMTP servers
     MUST NOT send reply codes whose first digits are other than 2, 3,4,
2624
2625
2626
2627

2629

















































  Internet-Draft        RFC5321-Numbered               Dec 2008
2631
2632
     or 5.  Clients that receive such out-of-range codes SHOULD normally
     treat them as fatal errors and terminate the mail transaction.
2635
  4.2.1.  Reply Code Severities and Theory
2637
     The three digits of the reply each have a special significance.  The
     first digit denotes whether the response is good, bad, or incomplete.
     An unsophisticated SMTP client, or one that receives an unexpected
     code, will be able to determine its next action (proceed as planned,
     redo, retrench, etc.) by examining this first digit.  An SMTP client
     that wants to know approximately what kind of error occurred (e.g.,
     mail system error, command syntax error) may examine the second
     digit.  The third digit and any supplemental information that maybe
     present is reserved for the finest gradation of information.
2647
     There are four values for the first digit of the reply code:
2649
     2yz  Positive Completion reply
        The requested action has been successfully completed.  A new
        request may be initiated.
2653
     3yz  Positive Intermediate reply
        The command has been accepted, but the requested action is being
        held in abeyance, pending receipt of further information.  The
        SMTP client should send another command specifying this
        information.  This reply is used in command sequence groups (i.e.,
        in DATA).
2660
     4yz  Transient Negative Completion reply
        The command was not accepted, and the requested action did not
        occur.  However, the error condition is temporary, and the action
        may be requested again.  The sender should return to the beginning
        of the command sequence (if any).  It is difficult to assign a
        meaning to "transient" when two different sites (receiver- and
        sender-SMTP agents) must agree on the interpretation.  Each reply
        in this category might have a different time value, but the SMTP
        client SHOULD try again.  A rule of thumb to determine whethera
        reply fits into the 4yz or the 5yz category (see below) is that
        replies are 4yz if they can be successful if repeated without


any
        change in command form or in properties of the sender or receiver
        (that is, the command is repeated identically and the receiver
        does not put up a new implementation).
2675
     5yz  Permanent Negative Completion reply
        The command was not accepted and the requested action did not
        occur.  The SMTP client SHOULD NOT repeat the exact request (in
        the same sequence).  Even some "permanent" error conditions can be
        corrected, so the human user may want to direct the SMTP client to
2681
2682
2683

2685











































  Internet-Draft        RFC5321-Numbered               Dec 2008
2687
2688
        reinitiate the command sequence by direct action at some pointin
        the future (e.g., after the spelling has been changed, or the user
        has altered the account status).
2692
     It is worth noting that the file transfer protocol (FTP) [34] uses a
     very similar code architecture and that the SMTP codes are based on
     the FTP model.  However, SMTP uses a one-command, one-response model
     (while FTP is asynchronous) and FTP's 1yz codes are not part of the
     SMTP model.
2698
     The second digit encodes responses in specific categories:
2700
     x0z  Syntax: These replies refer to syntax errors, syntactically
        correct commands that do not fit any functional category, and
        unimplemented or superfluous commands.
2704
     x1z  Information: These are replies to requests for information, such
        as status or help.
2707
     x2z  Connections: These are replies referring to the transmission
        channel.
2710
     x3z  Unspecified.
2712
     x4z  Unspecified.
2714
     x5z  Mail system: These replies indicate the status of the receiver
        mail system vis-a-vis the requested transfer or other mail system
        action.
2718
     The third digit gives a finer gradation of meaning in each category
     specified by the second digit.  The list of replies illustrates this.
     Each reply text is recommended rather than mandatory, and may even
     change according to the command with which it is associated.  On the
     other hand, the reply codes must strictly follow the specifications
     in this section.  Receiver implementations should not invent new
     codes for slightly different situations from the ones described here,
     but rather adapt codes already defined.
2727
     For example, a command such as NOOP, whose successful execution d


oes
     not offer the SMTP client any new information, will return a 250
     reply.  The reply is 502 when the command requests an unimplemented
     non-site-specific action.  A refinement of that is the 504 reply for
     a command that is implemented, but that requests an unimplemented
     parameter.
2734
2735
2736
2737
2738
2739

2741














































  Internet-Draft        RFC5321-Numbered               Dec 2008
2743
2744
     The reply text may be longer than a single line; in these cases the
     complete text must be marked so the SMTP client knows when it can
     stop reading the reply.  This requires a special format to indicate a
     multiple line reply.
2749
     The format for multiline replies requires that every line, exceptthe
     last, begin with the reply code, followed immediately by a hyphen,
     "-" (also known as minus), followed by text.  The last line will
     begin with the reply code, followed immediately by <SP>, optionally
     some text, and <CRLF>.  As noted above, servers SHOULD send the <SP>
     if subsequent text is not sent, but clients MUST be prepared for it
     to be omitted.
2757
     For example:
2759
        250-First line
        250-Second line
        250-234 Text beginning with numbers
        250 The last line
2764
     In a multiline reply, the reply code on each of the lines MUST bethe
     same.  It is reasonable for the client to rely on this, so it can
     make processing decisions based on the code in any line, assuming
     that all others will be the same.  In a few cases, there is important
     data for the client in the reply "text".  The client will be ableto
     identify these cases from the current context.
2771
  4.2.2.  Reply Codes by Function Groups
2773
     500  Syntax error, command unrecognized (This may include errors such
        as command line too long)
2776
     501  Syntax error in parameters or arguments
2778
     502  Command not implemented (see Section 4.2.4)
2780
     503  Bad sequence of commands
2782
     504  Command parameter not implemented
2784
2785
     211  System status, or system help reply
2787
     214  Help message (Information on how to use the receiver or the


        meaning of a particular non-standard command; this reply is useful
        only to the human user)
2791
2792
2793
2794
2795

2797




















































  Internet-Draft        RFC5321-Numbered               Dec 2008
2799
2800
     220  <domain> Service ready
2802
     221  <domain> Service closing transmission channel
2804
     421  <domain> Service not available, closing transmission channel
        (This may be a reply to any command if the service knows it must
        shut down)
2808
2809
     250  Requested mail action okay, completed
2811
     251  User not local; will forward to <forward-path> (See Section 3.4)
2813
     252  Cannot VRFY user, but will accept message and attempt delivery
        (See Section 3.5.3)
2816
     455  Server unable to accommodate parameters
2818
     555  MAIL FROM/RCPT TO parameters not recognized or not implemented
2820
     450  Requested mail action not taken: mailbox unavailable (e.g.,
        mailbox busy or temporarily blocked for policy reasons)
2823
     550  Requested action not taken: mailbox unavailable (e.g., mailbox
        not found, no access, or command rejected for policy reasons)
2826
     451  Requested action aborted: error in processing
2828
     551  User not local; please try <forward-path> (See Section 3.4)
2830
     452  Requested action not taken: insufficient system storage
2832
     552  Requested mail action aborted: exceeded storage allocation
2834
     553  Requested action not taken: mailbox name not allowed (e.g.,
        mailbox syntax incorrect)
2837
     354  Start mail input; end with <CRLF>.<CRLF>
2839
     554  Transaction failed (Or, in the case of a connection-opening
        response, "No SMTP service here")
2842
2843
2844
2845
2846
2847
2848
2849
2850


2851

2853



























































  Internet-Draft        RFC5321-Numbered               Dec 2008
2855
2856
  4.2.3.  Reply Codes in Numeric Order
2858
     211  System status, or system help reply
2860
     214  Help message (Information on how to use the receiver or the
        meaning of a particular non-standard command; this reply is useful
        only to the human user)
2864
     220  <domain> Service ready
2866
     221  <domain> Service closing transmission channel
2868
     250  Requested mail action okay, completed
2870
     251  User not local; will forward to <forward-path> (See Section 3.4)
2872
     252  Cannot VRFY user, but will accept message and attempt delivery
        (See Section 3.5.3)
2875
     354  Start mail input; end with <CRLF>.<CRLF>
2877
     421  <domain> Service not available, closing transmission channel
        (This may be a reply to any command if the service knows it must
        shut down)
2881
     450  Requested mail action not taken: mailbox unavailable (e.g.,
        mailbox busy or temporarily blocked for policy reasons)
2884
     451  Requested action aborted: local error in processing
2886
     452  Requested action not taken: insufficient system storage
2888
     455  Server unable to accommodate parameters
2890
     500  Syntax error, command unrecognized (This may include errors such
        as command line too long)
2893
     501  Syntax error in parameters or arguments
2895
     502  Command not implemented (see Section 4.2.4)
2897
     503  Bad sequence of commands
2899
     504  Command parameter not implemented
2901
     550  Requested action not taken: mailbox unavailable (e.g., mailbox
        not found, no access, or command rejected for policy reasons)
2904
2905


2906
2907

2909


























































  Internet-Draft        RFC5321-Numbered               Dec 2008
2911
2912
     551  User not local; please try <forward-path> (See Section 3.4)
2914
     552  Requested mail action aborted: exceeded storage allocation
2916
     553  Requested action not taken: mailbox name not allowed (e.g.,
        mailbox syntax incorrect)
2919
     554  Transaction failed (Or, in the case of a connection-opening
        response, "No SMTP service here")
2922
     555  MAIL FROM/RCPT TO parameters not recognized or not implemented
2924
  4.2.4.  Reply Code 502
2926
     Questions have been raised as to when reply code 502 (Command not
     implemented) SHOULD be returned in preference to other codes. 502
     SHOULD be used when the command is actually recognized by the SMTP
     server, but not implemented.  If the command is not recognized, code
     500 SHOULD be returned.  Extended SMTP systems MUST NOT list
     capabilities in response to EHLO for which they will return 502 (or
     500) replies.
2934
  4.2.5.  Reply Codes after DATA and the Subsequent <CRLF>.<CRLF>
2936
     When an SMTP server returns a positive completion status (2yz code)
     after the DATA command is completed with <CRLF>.<CRLF>, it accepts
     responsibility for:
2940
     o  delivering the message (if the recipient mailbox exists), or
2942
     o  if attempts to deliver the message fail due to transient
        conditions, retrying delivery some reasonable number of times at
        intervals as specified in Section 4.5.4.
2946
     o  if attempts to deliver the message fail due to permanent
        conditions, or if repeated attempts to deliver the message fail
        due to transient conditions, returning appropriate notification to
        the sender of the original message (using the address in the SMTP
        MAIL command).
2952
     When an SMTP server returns a temporary error status (4yz) code after
     the DATA command is completed with <CRLF>.<CRLF>, it MUST NOT make a
     subsequent attempt to deliver that message.  The SMTP client reta


ins
     responsibility for the delivery of that message and may either return
     it to the user or requeue it for a subsequent attempt (see
     Section 4.5.4.1).
2959
2960
2961
2962
2963

2965


















































  Internet-Draft        RFC5321-Numbered               Dec 2008
2967
2968
     The user who originated the message SHOULD be able to interpret the
     return of a transient failure status (by mail message or otherwise)
     as a non-delivery indication, just as a permanent failure would be
     interpreted.  If the client SMTP successfully handles these
     conditions, the user will not receive such a reply.
2974
     When an SMTP server returns a permanent error status (5yz) code after
     the DATA command is completed with <CRLF>.<CRLF>, it MUST NOT make
     any subsequent attempt to deliver the message.  As with temporary
     error status codes, the SMTP client retains responsibility for the
     message, but SHOULD not again attempt delivery to the same server
     without user review of the message and response and appropriate
     intervention.
2982
  4.3.  Sequencing of Commands and Replies
2984
  4.3.1.  Sequencing Overview
2986
     The communication between the sender and receiver is an alternating
     dialogue, controlled by the sender.  As such, the sender issues a
     command and the receiver responds with a reply.  Unless other
     arrangements are negotiated through service extensions, the sender
     MUST wait for this response before sending further commands.  One
     important reply is the connection greeting.  Normally, a receiver
     will send a 220 "Service ready" reply when the connection is
     completed.  The sender SHOULD wait for this greeting message before
     sending any commands.
2996
     Note: all the greeting-type replies have the official name (the
     fully-qualified primary domain name) of the server host as the first
     word following the reply code.  Sometimes the host will have no
     meaningful name.  See Section 4.1.3 for a discussion of alternatives
     in these situations.
3002
     For example,
3004
        220 ISIF.USC.EDU Service ready
3006
     or
3008
        220 mail.example.com SuperSMTP v 6.1.2 Service ready
3010
     or
3012


        220 [10.0.0.1] Clueless host service ready
3014
     The table below lists alternative success and failure replies for
     each command.  These SHOULD be strictly adhered to.  A receiver MAY
3017
3018
3019

3021




















































  Internet-Draft        RFC5321-Numbered               Dec 2008
3023
3024
     substitute text in the replies, but the meanings and actions implied
     by the code numbers and by the specific command reply sequence MUST
     be preserved.
3028
  4.3.2.  Command-Reply Sequences
3030
     Each command is listed with its usual possible replies.  The prefixes
     used before the possible replies are "I" for intermediate, "S" for
     success, and "E" for error.  Since some servers may generate other
     replies under special circumstances, and to allow for future
     extension, SMTP clients SHOULD, when possible, interpret only the
     first digit of the reply and MUST be prepared to deal with
     unrecognized reply codes by interpreting the first digit only.
     Unless extended using the mechanisms described in Section 2.2, SMTP
     servers MUST NOT transmit reply codes to an SMTP client that are
     other than three digits or that do not start in a digit between 2and
     5 inclusive.
3042
     These sequencing rules and, in principle, the codes themselves, can
     be extended or modified by SMTP extensions offered by the server and
     accepted (requested) by the client.  However, if the target is more
     precise granularity in the codes, rather than codes for completely
     new purposes, the system described in RFC 3463 [25] SHOULD be used in
     preference to the invention of new codes.
3049
     In addition to the codes listed below, any SMTP command can return
     any of the following codes if the corresponding unusual circumstances
     are encountered:
3053
     500  For the "command line too long" case or if the command name was
        not recognized.  Note that producing a "command not recognized"
        error in response to the required subset of these commands is a
        violation of this specification.  Similarly, producing a "command
        too long" message for a command line shorter than 512 characters
        would violate the provisions of Section 4.5.3.1.4.
3060


     501  Syntax error in command or arguments.  In order to provide for
        future extensions, commands that are specified in this document as
        not accepting arguments (DATA, RSET, QUIT) SHOULD return a 501
        message if arguments are supplied in the absence of EHLO-
        advertised extensions.
3066
     421  Service shutting down and closing transmission channel
3068
3069
3070
3071
3072
3073
3074
3075

3077











































  Internet-Draft        RFC5321-Numbered               Dec 2008
3079
3080
     Specific sequences are:
3082
        CONNECTION ESTABLISHMENT
3084
           S: 220
           E: 554
3087
        EHLO or HELO
3089
           S: 250
           E: 504 (a conforming implementation could return this code only
           in fairly obscure cases), 550, 502 (permitted only with an old-
           style server that does not support EHLO)
3094
        MAIL
3096
           S: 250
           E: 552, 451, 452, 550, 553, 503, 455, 555
3099
        RCPT
3101
           S: 250, 251 (but see Section 3.4 for discussion of 251 and 551)
           E: 550, 551, 552, 553, 450, 451, 452, 503, 455, 555
3104
        DATA
3106
           I: 354 -> data -> S: 250
3108
                             E: 552, 554, 451, 452
3110
                             E: 450, 550 (rejections for policy reasons)
3112
           E: 503, 554
3114
        RSET
3116
           S: 250
3118
        VRFY
3120
           S: 250, 251, 252
           E: 550, 551, 553, 502, 504
3123
        EXPN
3125
           S: 250, 252
           E: 550, 500, 502, 504
3128
3129
3130
3131



3133




























































  Internet-Draft        RFC5321-Numbered               Dec 2008
3135
3136
        HELP
3138
           S: 211, 214
           E: 502, 504
3141
        NOOP
3143
           S: 250
3145
        QUIT
3147
           S: 221
3149
  4.4.  Trace Information
3151
     When an SMTP server receives a message for delivery or further
     processing, it MUST insert trace ("time stamp" or "Received")
     information at the beginning of the message content, as discussedin
     Section 4.1.1.4.
3156
     This line MUST be structured as follows:
3158
     o  The FROM clause, which MUST be supplied in an SMTP environment,
        SHOULD contain both (1) the name of the source host as presented
        in the EHLO command and (2) an address literal containing the IP
        address of the source, determined from the TCP connection.
3163
     o  The ID clause MAY contain an "@" as suggested in RFC 822, but this
        is not required.
3166
     o  If the FOR clause appears, it MUST contain exactly one <path>
        entry, even when multiple RCPT commands have been given.  Multiple
        <path>s raise some security issues and have been deprecated, see
        Section 7.2.
3171
     An Internet mail program MUST NOT change or delete a Received: line
     that was previously added to the message header section.  SMTP
     servers MUST prepend Received lines to messages; they MUST NOT change
     the order of existing lines or insert Received lines in any other
     location.
3177
     As the Internet grows, comparability of Received header fields is
     important for detecting problems, especially slow relays.  SMTP
     servers that create Received header fields SHOULD use explicit
     offsets in the dates (e.g., -0800), rather than time zone names of


     any type.  Local time (with an offset) SHOULD be used rather thanUT
     when feasible.  This formulation allows slightly more information
     about local circumstances to be specified.  If UT is needed, the
3185
3186
3187

3189





















































  Internet-Draft        RFC5321-Numbered               Dec 2008
3191
3192
     receiver need merely do some simple arithmetic to convert the values.
     Use of UT loses information about the time zone-location of the
     server.  If it is desired to supply a time zone name, it SHOULD be
     included in a comment.
3197
     When the delivery SMTP server makes the "final delivery" of a
     message, it inserts a return-path line at the beginning of the mail
     data.  This use of return-path is required; mail systems MUST support
     it.  The return-path line preserves the information in the <reverse-
     path> from the MAIL command.  Here, final delivery means the message
     has left the SMTP environment.  Normally, this would mean it had been
     delivered to the destination user or an associated mail drop, butin
     some cases it may be further processed and transmitted by another
     mail system.
3207
     It is possible for the mailbox in the return path to be different
     from the actual sender's mailbox, for example, if error responsesare
     to be delivered to a special error handling mailbox rather than to
     the message sender.  When mailing lists are involved, this
     arrangement is common and useful as a means of directing errors to
     the list maintainer rather than the message originator.
3214
     The text above implies that the final mail data will begin with a
     return path line, followed by one or more time stamp lines.  These
     lines will be followed by the rest of the mail data: first the
     balance of the mail header section and then the body (RFC 5322 [4]).
3219
     It is sometimes difficult for an SMTP server to determine whetheror
     not it is making final delivery since forwarding or other operations
     may occur after the message is accepted for delivery.  Consequently,
     any further (forwarding, gateway, or relay) systems MAY remove the
     return path and rebuild the MAIL command as needed to ensure that
     exactly one such line appears in a delivered message.
3226
     A message-originating SMTP system SHOULD NOT send a message that
     already contains a Return-path header field.  SMTP servers performing
     a relay function MUST NOT inspect the message data, and especiall


y
     not to the extent needed to determine if Return-path header fields
     are present.  SMTP servers making final delivery MAY remove Return-
     path header fields before adding their own.
3233
     The primary purpose of the Return-path is to designate the address to
     which messages indicating non-delivery or other mail system failures
     are to be sent.  For this to be unambiguous, exactly one return path
     SHOULD be present when the message is delivered.  Systems using RFC
     822 syntax with non-SMTP transports SHOULD designate an unambiguous
     address, associated with the transport envelope, to which error
     reports (e.g., non-delivery messages) should be sent.
3241
3242
3243

3245






































  Internet-Draft        RFC5321-Numbered               Dec 2008
3247
3248
     Historical note: Text in RFC 822 that appears to contradict the use
     of the Return-path header field (or the envelope reverse-path address
     from the MAIL command) as the destination for error messages is not
     applicable on the Internet.  The reverse-path address (as copied into
     the Return-path) MUST be used as the target of any mail containing
     delivery error messages.
3255
     In particular:
     o  a gateway from SMTP -> elsewhere SHOULD insert a return-path
        header field, unless it is known that the "elsewhere" transport
        also uses Internet domain addresses and maintains the envelope
        sender address separately.
3261
     o  a gateway from elsewhere -> SMTP SHOULD delete any return-path
        header field present in the message, and either copy that
        information to the SMTP envelope or combine it with information
        present in the envelope of the other transport system to construct
        the reverse-path argument to the MAIL command in the SMTP
        envelope.
3268
     The server must give special treatment to cases in which the
     processing following the end of mail data indication is only
     partially successful.  This could happen if, after accepting several
     recipients and the mail data, the SMTP server finds that the mail
     data could be successfully delivered to some, but not all, of the
     recipients.  In such cases, the response to the DATA command MUSTbe
     an OK reply.  However, the SMTP server MUST compose and send an
     "undeliverable mail" notification message to the originator of the
     message.
3278
     A single notification listing all of the failed recipients or
     separate notification messages MUST be sent for each failed
     recipient.  For economy of processing by the sender, the former
     SHOULD be used when possible.  Note that the key difference between
     handling aliases (Section 3.9.1) and forwarding (this subsection)is
     the change to the backward-pointing address in this case.  All
     notification messages about undeliverable mail MUST be sent usingthe
     MAIL command (even if they result from processing the obsolete SEND,
     SOML, or SAML commands) and MUST use a null return path as discussed


     in Section 3.6.
3289
     The time stamp line and the return path line are formally definedas
     follows (the definitions for "FWS" and "CFWS" appear in RFC 5322
     [4]):
3293
     Return-path-line  = "Return-Path:" FWS Reverse-path <CRLF>
3295
     Time-stamp-line  = "Received:" FWS Stamp <CRLF>
3297
3298
3299

3301















































  Internet-Draft        RFC5321-Numbered               Dec 2008
3303
3304
     Stamp          = From-domain By-domain Opt-info [CFWS] ";"
                    FWS date-time
                    ; where "date-time" is as defined in RFC 5322 [4]
                    ; but the "obs-" forms, especially two-digit
                    ; years, are prohibited in SMTP and MUST NOT be used.
3310
     From-domain    = "FROM" FWS Extended-Domain
3312
     By-domain      = CFWS "BY" FWS Extended-Domain
3314
     Extended-Domain  = Domain /
                      ( Domain FWS "(" TCP-info ")" ) /
                      ( address-literal FWS "(" TCP-info ")" )
3318
     TCP-info       = address-literal / ( Domain FWS address-literal )
                    ; Information derived by server from TCP connection
                    ; not client EHLO.
3322
     Opt-info       = [Via] [With] [ID] [For]
                    [Additional-Registered-Clauses]
3325
     Via            = CFWS "VIA" FWS Link
3327
     With           = CFWS "WITH" FWS Protocol
3329
     ID             = CFWS "ID" FWS ( Atom / msg-id )
                    ; msg-id is defined in RFC 5322 [4]
3332
     For            = CFWS "FOR" FWS ( Path / Mailbox )
3334
     Additional-Registered-Clauses  = CFWS Atom FWS String
                                    ; Additional standard clauses may be
                                    added in this
                                    ; location by future standards and
                                    registration with
                                    ; IANA.  SMTP servers SHOULD NOT use
                                    unregistered
                                    ; names.  See Section 8.
3343
     Link           = "TCP" / Addtl-Link
3345
     Addtl-Link     = Atom
                    ; Additional standard names for links are
                    ; registered with the Internet Assigned Numbers
                    ; Authority (IANA).  "Via" is primarily of value
                    ; with non-Internet transports.  SMTP servers
                    ; SHOULD NOT use unregistered names.
3352
3353
3354
3355



3357




























































  Internet-Draft        RFC5321-Numbered               Dec 2008
3359
3360
     Protocol       = "ESMTP" / "SMTP" / Attdl-Protocol
3362
     Attdl-Protocol = Atom
                    ; Additional standard names for protocols are
                    ; registered with the Internet Assigned Numbers
                    ; Authority (IANA) in the "mail parameters"
                    ; registry [9].  SMTP servers SHOULD NOT
                    ; use unregistered names.
3369
  4.5.  Additional Implementation Issues
3371
  4.5.1.  Minimum Implementation
3373
     In order to make SMTP workable, the following minimum implementation
     MUST be provided by all receivers.  The following commands MUST be
     supported to conform to this specification:
3377
        EHLO
        HELO
        MAIL
        RCPT
        DATA
        RSET
        NOOP
        QUIT
        VRFY
3387
     Any system that includes an SMTP server supporting mail relaying or
     delivery MUST support the reserved mailbox "postmaster" as a case-
     insensitive local name.  This postmaster address is not strictly
     necessary if the server always returns 554 on connection opening (as
     described in Section 3.1).  The requirement to accept mail for
     postmaster implies that RCPT commands that specify a mailbox for
     postmaster at any of the domains for which the SMTP server provides
     mail service, as well as the special case of "RCPT TO:<Postmaster>"
     (with no domain specification), MUST be supported.
3397
     SMTP systems are expected to make every reasonable effort to accept
     mail directed to Postmaster from any other system on the Internet.
     In extreme cases -- such as to contain a denial of service attackor
     other breach of security -- an SMTP server may block mail directed to
     Postmaster.  However, such arrangements SHOULD be narrowly tailored
     so as to avoid blocking messages that are not part of such attack


s.
3404
3405
3406
3407
3408
3409
3410
3411

3413



















































  Internet-Draft        RFC5321-Numbered               Dec 2008
3415
3416
  4.5.2.  Transparency
3418
     Without some provision for data transparency, the character sequence
     "<CRLF>.<CRLF>" ends the mail text and cannot be sent by the user.
     In general, users are not aware of such "forbidden" sequences.  To
     allow all user composed text to be transmitted transparently, the
     following procedures are used:
3424
     o  Before sending a line of mail text, the SMTP client checks the
        first character of the line.  If it is a period, one additional
        period is inserted at the beginning of the line.
3428
     o  When a line of mail text is received by the SMTP server, it checks
        the line.  If the line is composed of a single period, it is
        treated as the end of mail indicator.  If the first character is a
        period and there are other characters on the line, the first
        character is deleted.
3434
     The mail data may contain any of the 128 ASCII characters.  All
     characters are to be delivered to the recipient's mailbox, including
     spaces, vertical and horizontal tabs, and other control characters.
     If the transmission channel provides an 8-bit byte (octet) data
     stream, the 7-bit ASCII codes are transmitted, right justified, in
     the octets, with the high-order bits cleared to zero.  See
     Section 3.6 for special treatment of these conditions in SMTP systems
     serving a relay function.
3443
     In some systems, it may be necessary to transform the data as it is
     received and stored.  This may be necessary for hosts that use a
     different character set than ASCII as their local character set, that
     store data in records rather than strings, or which use special
     character sequences as delimiters inside mailboxes.  If such
     transformations are necessary, they MUST be reversible, especially if
     they are applied to mail being relayed.
3451
  4.5.3.  Sizes and Timeouts
3453
  4.5.3.1.  Size Limits and Minimums
3455
     There are several objects that have required minimum/maximum sizes.
     Every implementation MUST be able to receive objects of at least


     these sizes.  Objects larger than these sizes SHOULD be avoided when
     possible.  However, some Internet mail constructs such as encoded
     X.400 addresses (RFC 2156 [35]) will often require larger objects.
     Clients MAY attempt to transmit these, but MUST be prepared for a
     server to reject them if they cannot be handled by it.  To the
     maximum extent possible, implementation techniques that impose no
     limits on the length of these objects should be used.
3465
3466
3467

3469
















































  Internet-Draft        RFC5321-Numbered               Dec 2008
3471
3472
     Extensions to SMTP may involve the use of characters that occupy more
     than a single octet each.  This section therefore specifies lengths
     in octets where absolute lengths, rather than character counts, are
     intended.
3477
  4.5.3.1.1.  Local-part
3479
     The maximum total length of a user name or other local-part is 64
     octets.
3482
  4.5.3.1.2.  Domain
3484
     The maximum total length of a domain name or number is 255 octets.
3486
  4.5.3.1.3.  Path
3488
     The maximum total length of a reverse-path or forward-path is 256
     octets (including the punctuation and element separators).
3491
  4.5.3.1.4.  Command Line
3493
     The maximum total length of a command line including the command word
     and the <CRLF> is 512 octets.  SMTP extensions may be used to
     increase this limit.
3497
  4.5.3.1.5.  Reply Line
3499
     The maximum total length of a reply line including the reply codeand
     the <CRLF> is 512 octets.  More information may be conveyed through
     multiple-line replies.
3503
  4.5.3.1.6.  Text Line
3505
     The maximum total length of a text line including the <CRLF> is 1000
     octets (not counting the leading dot duplicated for transparency).
     This number may be increased by the use of SMTP Service Extensions.
3509
  4.5.3.1.7.  Message Content
3511
     The maximum total length of a message content (including any message
     header section as well as the message body) MUST BE at least 64K
     octets.  Since the introduction of Internet Standards for multimedia
     mail (RFC 2045 [21]), message lengths on the Internet have grown


     dramatically, and message size restrictions should be avoided if at
     all possible.  SMTP server systems that must impose restrictions
     SHOULD implement the "SIZE" service extension of RFC 1870 [10], and
     SMTP client systems that will send large messages SHOULD utilize it
     when possible.
3521
3522
3523

3525

















































  Internet-Draft        RFC5321-Numbered               Dec 2008
3527
3528
  4.5.3.1.8.  Recipients Buffer
3530
     The minimum total number of recipients that MUST be buffered is 100
     recipients.  Rejection of messages (for excessive recipients) with
     fewer than 100 RCPT commands is a violation of this specification.
     The general principle that relaying SMTP server MUST NOT, and
     delivery SMTP servers SHOULD NOT, perform validation tests on message
     header fields suggests that messages SHOULD NOT be rejected basedon
     the total number of recipients shown in header fields.  A server that
     imposes a limit on the number of recipients MUST behave in an orderly
     fashion, such as rejecting additional addresses over its limit rather
     than silently discarding addresses previously accepted.  A client
     that needs to deliver a message containing over 100 RCPT commands
     SHOULD be prepared to transmit in 100-recipient "chunks" if the
     server declines to accept more than 100 recipients in a single
     message.
3545
  4.5.3.1.9.  Treatment When Limits Exceeded
3547
     Errors due to exceeding these limits may be reported by using the
     reply codes.  Some examples of reply codes are:
3550
        500 Line too long.
3552
     or
3554
        501 Path too long
3556
     or
3558
        452 Too many recipients (see below)
3560
     or
3562
        552 Too much mail data.
3564
  4.5.3.1.10.  Too Many Recipients Code
3566
     RFC 821 [1] incorrectly listed the error where an SMTP server
     exhausts its implementation limit on the number of RCPT commands
     ("too many recipients") as having reply code 552.  The correct reply
     code for this condition is 452.  Clients SHOULD treat a 552 code in
     this case as a temporary, rather than permanent, failure so the logic
     below works.


3573
     When a conforming SMTP server encounters this condition, it has at
     least 100 successful RCPT commands in its recipients buffer.  If the
     server is able to accept the message, then at least these 100
3577
3578
3579

3581



















































  Internet-Draft        RFC5321-Numbered               Dec 2008
3583
3584
     addresses will be removed from the SMTP client's queue.  When the
     client attempts retransmission of those addresses that received 452
     responses, at least 100 of these will be able to fit in the SMTP
     server's recipients buffer.  Each retransmission attempt that is able
     to deliver anything will be able to dispose of at least 100 of these
     recipients.
3591
     If an SMTP server has an implementation limit on the number of RCPT
     commands and this limit is exhausted, it MUST use a response codeof
     452 (but the client SHOULD also be prepared for a 552, as noted
     above).  If the server has a configured site-policy limitation onthe
     number of RCPT commands, it MAY instead use a 5yz response code. In
     particular, if the intent is to prohibit messages with more than a
     site-specified number of recipients, rather than merely limit the
     number of recipients in a given mail transaction, it would be
     reasonable to return a 503 response to any DATA command received
     subsequent to the 452 (or 552) code or to simply return the 503 after
     DATA without returning any previous negative response.
3603
  4.5.3.2.  Timeouts
3605
     An SMTP client MUST provide a timeout mechanism.  It MUST use per-
     command timeouts rather than somehow trying to time the entire mail
     transaction.  Timeouts SHOULD be easily reconfigurable, preferably
     without recompiling the SMTP code.  To implement this, a timer isset
     for each SMTP command and for each buffer of the data transfer.  The
     latter means that the overall timeout is inherently proportional to
     the size of the message.
3613
     Based on extensive experience with busy mail-relay hosts, the minimum
     per-command timeout values SHOULD be as follows:
3616
  4.5.3.2.1.  Initial 220 Message: 5 Minutes
3618
     An SMTP client process needs to distinguish between a failed TCP
     connection and a delay in receiving the initial 220 greeting message.
     Many SMTP servers accept a TCP connection but delay delivery of the


     220 message until their system load permits more mail to be
     processed.
3624
  4.5.3.2.2.  MAIL Command: 5 Minutes
3626
  4.5.3.2.3.  RCPT Command: 5 Minutes
3628
     A longer timeout is required if processing of mailing lists and
     aliases is not deferred until after the message was accepted.
3631
3632
3633
3634
3635

3637














































  Internet-Draft        RFC5321-Numbered               Dec 2008
3639
3640
  4.5.3.2.4.  DATA Initiation: 2 Minutes
3642
     This is while awaiting the "354 Start Input" reply to a DATA command.
3644
  4.5.3.2.5.  Data Block: 3 Minutes
3646
     This is while awaiting the completion of each TCP SEND call
     transmitting a chunk of data.
3649
  4.5.3.2.6.  DATA Termination: 10 Minutes.
3651
     This is while awaiting the "250 OK" reply.  When the receiver gets
     the final period terminating the message data, it typically performs
     processing to deliver the message to a user mailbox.  A spurious
     timeout at this point would be very wasteful and would typically
     result in delivery of multiple copies of the message, since it has
     been successfully sent and the server has accepted responsibilityfor
     delivery.  See Section 6.1 for additional discussion.
3659
  4.5.3.2.7.  Server Timeout: 5 Minutes.
3661
     An SMTP server SHOULD have a timeout of at least 5 minutes while it
     is awaiting the next command from the sender.
3664
  4.5.4.  Retry Strategies
3666
     The common structure of a host SMTP implementation includes user
     mailboxes, one or more areas for queuing messages in transit, andone
     or more daemon processes for sending and receiving mail.  The exact
     structure will vary depending on the needs of the users on the host
     and the number and size of mailing lists supported by the host.  We
     describe several optimizations that have proved helpful, particularly
     for mailers supporting high traffic levels.
3674
     Any queuing strategy MUST include timeouts on all activities on a
     per-command basis.  A queuing strategy MUST NOT send error messages
     in response to error messages under any circumstances.
3678
  4.5.4.1.  Sending Strategy
3680
     The general model for an SMTP client is one or more processes that
     periodically attempt to transmit outgoing mail.  In a typical sys


tem,
     the program that composes a message has some method for requesting
     immediate attention for a new piece of outgoing mail, while mail that
     cannot be transmitted immediately MUST be queued and periodically
     retried by the sender.  A mail queue entry will include not only the
     message itself but also the envelope information.
3688
3689
3690
3691

3693















































  Internet-Draft        RFC5321-Numbered               Dec 2008
3695
3696
     The sender MUST delay retrying a particular destination after one
     attempt has failed.  In general, the retry interval SHOULD be at
     least 30 minutes; however, more sophisticated and variable strategies
     will be beneficial when the SMTP client can determine the reason for
     non-delivery.
3702
     Retries continue until the message is transmitted or the sender gives
     up; the give-up time generally needs to be at least 4-5 days.  ItMAY
     be appropriate to set a shorter maximum number of retries for non-
     delivery notifications and equivalent error messages than for
     standard messages.  The parameters to the retry algorithm MUST be
     configurable.
3709
     A client SHOULD keep a list of hosts it cannot reach and
     corresponding connection timeouts, rather than just retrying queued
     mail items.
3713
     Experience suggests that failures are typically transient (the target
     system or its connection has crashed), favoring a policy of two
     connection attempts in the first hour the message is in the queue,
     and then backing off to one every two or three hours.
3718
     The SMTP client can shorten the queuing delay in cooperation withthe
     SMTP server.  For example, if mail is received from a particular
     address, it is likely that mail queued for that host can now be sent.
     Application of this principle may, in many cases, eliminate the
     requirement for an explicit "send queues now" function such as ETRN,
     RFC 1985 [36].
3725
     The strategy may be further modified as a result of multiple
     addresses per host (see below) to optimize delivery time versus
     resource usage.
3729
     An SMTP client may have a large queue of messages for each
     unavailable destination host.  If all of these messages were retried
     in every retry cycle, there would be excessive Internet overhead and
     the sending system would be blocked for a long period.  Note thatan
     SMTP client can generally determine that a delivery attempt has
     failed only after a timeout of several minutes, and even a one-minute
     timeout per connection will result in a very large delay if retri


es
     are repeated for dozens, or even hundreds, of queued messages to the
     same host.
3739
     At the same time, SMTP clients SHOULD use great care in caching
     negative responses from servers.  In an extreme case, if EHLO is
     issued multiple times during the same SMTP connection, different
     answers may be returned by the server.  More significantly, 5yz
     responses to the MAIL command MUST NOT be cached.
3745
3746
3747

3749















































  Internet-Draft        RFC5321-Numbered               Dec 2008
3751
3752
     When a mail message is to be delivered to multiple recipients, and
     the SMTP server to which a copy of the message is to be sent is the
     same for multiple recipients, then only one copy of the message
     SHOULD be transmitted.  That is, the SMTP client SHOULD use the
     command sequence: MAIL, RCPT, RCPT, ..., RCPT, DATA instead of the
     sequence: MAIL, RCPT, DATA, ..., MAIL, RCPT, DATA.  However, if there
     are very many addresses, a limit on the number of RCPT commands per
     MAIL command MAY be imposed.  This efficiency feature SHOULD be
     implemented.
3762
     Similarly, to achieve timely delivery, the SMTP client MAY support
     multiple concurrent outgoing mail transactions.  However, some limit
     may be appropriate to protect the host from devoting all its
     resources to mail.
3767
  4.5.4.2.  Receiving Strategy
3769
     The SMTP server SHOULD attempt to keep a pending listen on the SMTP
     port (specified by IANA as port 25) at all times.  This requires the
     support of multiple incoming TCP connections for SMTP.  Some limit
     MAY be imposed, but servers that cannot handle more than one SMTP
     transaction at a time are not in conformance with the intent of this
     specification.
3776
     As discussed above, when the SMTP server receives mail from a
     particular host address, it could activate its own SMTP queuing
     mechanisms to retry any mail pending for that host address.
3780
  4.5.5.  Messages with a Null Reverse-Path
3782
     There are several types of notification messages that are required by
     existing and proposed Standards to be sent with a null reverse-path,
     namely non-delivery notifications as discussed in Section 3.7, other
     kinds of Delivery Status Notifications (DSNs, RFC 3461 [32]), and
     Message Disposition Notifications (MDNs, RFC 3798 [37]).  All of
     these kinds of messages are notifications about a previous message,
     and they are sent to the reverse-path of the previous mail message.
     (If the delivery of such a notification message fails, that usually


     indicates a problem with the mail system of the host to which the
     notification message is addressed.  For this reason, at some hosts
     the MTA is set up to forward such failed notification messages to
     someone who is able to fix problems with the mail system, e.g., via
     the postmaster alias.)
3796
     All other types of messages (i.e., any message which is not required
     by a Standards-Track RFC to have a null reverse-path) SHOULD be sent
     with a valid, non-null reverse-path.
3800
3801
3802
3803

3805











































  Internet-Draft        RFC5321-Numbered               Dec 2008
3807
3808
     Implementers of automated email processors should be careful to make
     sure that the various kinds of messages with a null reverse-path are
     handled correctly.  In particular, such systems SHOULD NOT reply to
     messages with a null reverse-path, and they SHOULD NOT add a non-null
     reverse-path, or change a null reverse-path to a non-null one, to
     such messages when forwarding.
3815
  5.  Address Resolution and Mail Handling
3817
  5.1.  Locating the Target Host
3819
     Once an SMTP client lexically identifies a domain to which mail will
     be delivered for processing (as described in Sections 2.3.5 and 3.6),
     a DNS lookup MUST be performed to resolve the domain name (RFC 1035
     [2]).  The names are expected to be fully-qualified domain names
     (FQDNs): mechanisms for inferring FQDNs from partial names or local
     aliases are outside of this specification.  Due to a history of
     problems, SMTP servers used for initial submission of messages SHOULD
     NOT make such inferences (Message Submission Servers [18] have
     somewhat more flexibility) and intermediate (relay) SMTP servers MUST
     NOT make them.
3830
     The lookup first attempts to locate an MX record associated with the
     name.  If a CNAME record is found, the resulting name is processed as
     if it were the initial name.  If a non-existent domain error is
     returned, this situation MUST be reported as an error.  If a
     temporary error is returned, the message MUST be queued and retried
     later (see Section 4.5.4.1).  If an empty list of MXs is returned,
     the address is treated as if it was associated with an implicit MX
     RR, with a preference of 0, pointing to that host.  If MX recordsare
     present, but none of them are usable, or the implicit MX is unusable,
     this situation MUST be reported as an error.
3841
     If one or more MX RRs are found for a given name, SMTP systems MUST
     NOT utilize any address RRs associated with that name unless theyare
     located using the MX RRs; the "implicit MX" rule above applies on


ly
     if there are no MX records present.  If MX records are present, but
     none of them are usable, this situation MUST be reported as an error.
3847
     When a domain name associated with an MX RR is looked up and the
     associated data field obtained, the data field of that response MUST
     contain a domain name.  That domain name, when queried, MUST return
     at least one address record (e.g., A or AAAA RR) that gives the IP
     address of the SMTP server to which the message should be directed.
     Any other response, specifically including a value that will return a
     CNAME record when queried, lies outside the scope of this Standard.
     The prohibition on labels in the data that resolve to CNAMEs is
     discussed in more detail in RFC 2181, Section 10.3 [38].
3857
3858
3859

3861




































  Internet-Draft        RFC5321-Numbered               Dec 2008
3863
3864
     When the lookup succeeds, the mapping can result in a list of
     alternative delivery addresses rather than a single address, because
     of multiple MX records, multihoming, or both.  To provide reliable
     mail transmission, the SMTP client MUST be able to try (and retry)
     each of the relevant addresses in this list in order, until a
     delivery attempt succeeds.  However, there MAY also be a configurable
     limit on the number of alternate addresses that can be tried.  Inany
     case, the SMTP client SHOULD try at least two addresses.
3873
     Two types of information are used to rank the host addresses:
     multiple MX records, and multihomed hosts.
3876
     MX records contain a preference indication that MUST be used in
     sorting if more than one such record appears (see below).  Lower
     numbers are more preferred than higher ones.  If there are multiple
     destinations with the same preference and there is no clear reason to
     favor one (e.g., by recognition of an easily reached address), then
     the sender-SMTP MUST randomize them to spread the load across
     multiple mail exchangers for a specific organization.
3884
     The destination host (perhaps taken from the preferred MX record)may
     be multihomed, in which case the domain name resolver will returna
     list of alternative IP addresses.  It is the responsibility of the
     domain name resolver interface to have ordered this list by
     decreasing preference if necessary, and the SMTP sender MUST try them
     in the order presented.
3891
     Although the capability to try multiple alternative addresses is
     required, specific installations may want to limit or disable theuse
     of alternative addresses.  The question of whether a sender should
     attempt retries using the different addresses of a multihomed host
     has been controversial.  The main argument for using the multiple
     addresses is that it maximizes the probability of timely delivery,
     and indeed sometimes the probability of any delivery; the counter-
     argument is that it may result in unnecessary resource use.  Note
     that resource use is also strongly determined by the sending strategy
     discussed in Section 4.5.4.1.


3902
     If an SMTP server receives a message with a destination for whichit
     is a designated Mail eXchanger, it MAY relay the message (potentially
     after having rewritten the MAIL FROM and/or RCPT TO addresses), make
     final delivery of the message, or hand it off using some mechanism
     outside the SMTP-provided transport environment.  Of course, neither
     of the latter require that the list of MX records be examined
     further.
3910
     If it determines that it should relay the message without rewriting
     the address, it MUST sort the MX records to determine candidates for
3913
3914
3915

3917







































  Internet-Draft        RFC5321-Numbered               Dec 2008
3919
3920
     delivery.  The records are first ordered by preference, with the
     lowest-numbered records being most preferred.  The relay host MUST
     then inspect the list for any of the names or addresses by which it
     might be known in mail transactions.  If a matching record is found,
     all records at that preference level and higher-numbered ones MUST be
     discarded from consideration.  If there are no records left at that
     point, it is an error condition, and the message MUST be returnedas
     undeliverable.  If records do remain, they SHOULD be tried, best
     preference first, as described above.
3930
  5.2.  IPv6 and MX Records
3932
     In the contemporary Internet, SMTP clients and servers may be hosted
     on IPv4 systems, IPv6 systems, or dual-stack systems that are
     compatible with either version of the Internet Protocol.  The host
     domains to which MX records point may, consequently, contain "A RR"s
     (IPv4), "AAAA RR"s (IPv6), or any combination of them.  While RFC
     3974 [39] discusses some operational experience in mixed
     environments, it was not comprehensive enough to justify
     standardization, and some of its recommendations appear to be
     inconsistent with this specification.  The appropriate actions tobe
     taken either will depend on local circumstances, such as performance
     of the relevant networks and any conversions that might be necessary,
     or will be obvious (e.g., an IPv6-only client need not attempt to
     look up A RRs or attempt to reach IPv4-only servers).  Designers of
     SMTP implementations that might run in IPv6 or dual-stack
     environments should study the procedures above, especially the
     comments about multihomed hosts, and, preferably, provide mechanisms
     to facilitate operational tuning and mail interoperability between
     IPv4 and IPv6 systems while considering local circumstances.
3951
  6.  Problem Detection and Handling
3953
  6.1.  Reliable Delivery and Replies by Email
3955
     When the receiver-SMTP accepts a piece of mail (by sending a "250OK"
     message in response to DATA), it is accepting responsibility for
     delivering or relaying the message.  It must take this responsibility


     seriously.  It MUST NOT lose the message for frivolous reasons, such
     as because the host later crashes or because of a predictable
     resource shortage.  Some reasons that are not considered frivolous
     are discussed in the next subsection and in Section 7.8.
3963
     If there is a delivery failure after acceptance of a message, the
     receiver-SMTP MUST formulate and mail a notification message.  This
     notification MUST be sent using a null ("<>") reverse-path in the
     envelope.  The recipient of this notification MUST be the address
     from the envelope return path (or the Return-Path: line).  However,
3969
3970
3971

3973











































  Internet-Draft        RFC5321-Numbered               Dec 2008
3975
3976
     if this address is null ("<>"), the receiver-SMTP MUST NOT send a
     notification.  Obviously, nothing in this section can or should
     prohibit local decisions (i.e., as part of the same system
     environment as the receiver-SMTP) to log or otherwise transmit
     information about null address events locally if that is desired.If
     the address is an explicit source route, it MUST be stripped downto
     its final hop.
3984
     For example, suppose that an error notification must be sent for a
     message that arrived with:
3987
        MAIL FROM:<@a,@b:user@d>
3989
     The notification message MUST be sent using:
3991
        RCPT TO:<user@d>
3993
     Some delivery failures after the message is accepted by SMTP willbe
     unavoidable.  For example, it may be impossible for the receiving
     SMTP server to validate all the delivery addresses in RCPT command(s)
     due to a "soft" domain system error, because the target is a mailing
     list (see earlier discussion of RCPT), or because the server is
     acting as a relay and has no immediate access to the delivering
     system.
4001
     To avoid receiving duplicate messages as the result of timeouts, a
     receiver-SMTP MUST seek to minimize the time required to respond to
     the final <CRLF>.<CRLF> end of data indicator.  See RFC 1047 [40]for
     a discussion of this problem.
4006
  6.2.  Unwanted, Unsolicited, and "Attack" Messages
4008
     Utility and predictability of the Internet mail system requires that
     messages that can be delivered should be delivered, regardless ofany
     syntax or other faults associated with those messages and regardless
     of their content.  If they cannot be delivered, and cannot be
     rejected by the SMTP server during the SMTP transaction, they should
     be "bounced" (returned with non-delivery notification messages) as
     described above.  In today's world, in which many SMTP server
     operators have discovered that the quantity of undesirable bulk email


     vastly exceeds the quantity of desired mail and in which accepting a
     message may trigger additional undesirable traffic by providing
     verification of the address, those principles may not be practical.
4020
     As discussed in Section 7.8 and Section 7.9 below, dropping mail
     without notification of the sender is permitted in practice.
     However, it is extremely dangerous and violates a long tradition and
     community expectations that mail is either delivered or returned.If
4025
4026
4027

4029













































  Internet-Draft        RFC5321-Numbered               Dec 2008
4031
4032
     silent message-dropping is misused, it could easily undermine
     confidence in the reliability of the Internet's mail systems.  So
     silent dropping of messages should be considered only in those cases
     where there is very high confidence that the messages are seriously
     fraudulent or otherwise inappropriate.
4038
     To stretch the principle of delivery if possible even further, itmay
     be a rational policy to not deliver mail that has an invalid return
     address, although the history of the network is that users are
     typically better served by delivering any message that can be
     delivered.  Reliably determining that a return address is invalidcan
     be a difficult and time-consuming process, especially if the putative
     sending system is not directly accessible or does not fully and
     accurately support VRFY and, even if a "drop messages with invalid
     return addresses" policy is adopted, it SHOULD be applied only when
     there is near-certainty that the return addresses are, in fact,
     invalid.
4050
     Conversely, if a message is rejected because it is found to contain
     hostile content (a decision that is outside the scope of an SMTP
     server as defined in this document), rejection ("bounce") messages
     SHOULD NOT be sent unless the receiving site is confident that those
     messages will be usefully delivered.  The preference and default in
     these cases is to avoid sending non-delivery messages when the
     incoming message is determined to contain hostile content.
4058
  6.3.  Loop Detection
4060
     Simple counting of the number of "Received:" header fields in a
     message has proven to be an effective, although rarely optimal,
     method of detecting loops in mail systems.  SMTP servers using this
     technique SHOULD use a large rejection threshold, normally at least
     100 Received entries.  Whatever mechanisms are used, servers MUST
     contain provisions for detecting and stopping trivial loops.
4067
  6.4.  Compensating for Irregularities
4069
     Unfortunately, variations, creative interpretations, and outright
     violations of Internet mail protocols do occur; some would suggest
     that they occur quite frequently.  The debate as to whether a wel


l-
     behaved SMTP receiver or relay should reject a malformed message,
     attempt to pass it on unchanged, or attempt to repair it to increase
     the odds of successful delivery (or subsequent reply) began almost
     with the dawn of structured network mail and shows no signs of
     abating.  Advocates of rejection claim that attempted repairs are
     rarely completely adequate and that rejection of bad messages is the
     only way to get the offending software repaired.  Advocates of
     "repair" or "deliver no matter what" argue that users prefer that
4081
4082
4083

4085













































  Internet-Draft        RFC5321-Numbered               Dec 2008
4087
4088
     mail go through it if at all possible and that there are significant
     market pressures in that direction.  In practice, these market
     pressures may be more important to particular vendors than strict
     conformance to the standards, regardless of the preference of the
     actual developers.
4094
     The problems associated with ill-formed messages were exacerbatedby
     the introduction of the split-UA mail reading protocols (Post Office
     Protocol (POP) version 2 [15], Post Office Protocol (POP) version3
     [16], IMAP version 2 [41], and PCMAIL [42]).  These protocols
     encouraged the use of SMTP as a posting (message submission)
     protocol, and SMTP servers as relay systems for these client hosts
     (which are often only intermittently connected to the Internet).
     Historically, many of those client machines lacked some of the
     mechanisms and information assumed by SMTP (and indeed, by the mail
     format protocol, RFC 822 [28]).  Some could not keep adequate track
     of time; others had no concept of time zones; still others could not
     identify their own names or addresses; and, of course, none could
     satisfy the assumptions that underlay RFC 822's conception of
     authenticated addresses.
4109
     In response to these weak SMTP clients, many SMTP systems now
     complete messages that are delivered to them in incomplete or
     incorrect form.  This strategy is generally considered appropriate
     when the server can identify or authenticate the client, and there
     are prior agreements between them.  By contrast, there is at best
     great concern about fixes applied by a relay or delivery SMTP server
     that has little or no knowledge of the user or client machine.  Many
     of these issues are addressed by using a separate protocol, such as
     that defined in RFC 4409 [18], for message submission, rather than
     using originating SMTP servers for that purpose.
4120
     The following changes to a message being processed MAY be applied
     when necessary by an originating SMTP server, or one used as the
     target of SMTP as an initial posting (message submission) protocol:
4124
     o  Addition of a message-id field when none appears
4126
     o  Addition of a date, time, or time zone when none appears
4128


     o  Correction of addresses to proper FQDN format
4130
     The less information the server has about the client, the less likely
     these changes are to be correct and the more caution and conservatism
     should be applied when considering whether or not to perform fixes
     and how.  These changes MUST NOT be applied by an SMTP server that
     provides an intermediate relay function.
4136
4137
4138
4139

4141













































  Internet-Draft        RFC5321-Numbered               Dec 2008
4143
4144
     In all cases, properly operating clients supplying correct
     information are preferred to corrections by the SMTP server.  In all
     cases, documentation SHOULD be provided in trace header fields and/or
     header field comments for actions performed by the servers.
4149
  7.  Security Considerations
4151
  7.1.  Mail Security and Spoofing
4153
     SMTP mail is inherently insecure in that it is feasible for even
     fairly casual users to negotiate directly with receiving and relaying
     SMTP servers and create messages that will trick a naive recipient
     into believing that they came from somewhere else.  Constructing such
     a message so that the "spoofed" behavior cannot be detected by an
     expert is somewhat more difficult, but not sufficiently so as to be a
     deterrent to someone who is determined and knowledgeable.
     Consequently, as knowledge of Internet mail increases, so does the
     knowledge that SMTP mail inherently cannot be authenticated, or
     integrity checks provided, at the transport level.  Real mail
     security lies only in end-to-end methods involving the message
     bodies, such as those that use digital signatures (see RFC 1847 [43]
     and, e.g., Pretty Good Privacy (PGP) in RFC 4880 [44] or Secure/
     Multipurpose Internet Mail Extensions (S/MIME) in RFC 3851 [45]).
4168
     Various protocol extensions and configuration options that provide
     authentication at the transport level (e.g., from an SMTP client to
     an SMTP server) improve somewhat on the traditional situation
     described above.  However, in general, they only authenticate one
     server to another rather than a chain of relays and servers, much
     less authenticating users or user machines.  Consequently, unless
     they are accompanied by careful handoffs of responsibility in a
     carefully designed trust environment, they remain inherently weaker
     than end-to-end mechanisms that use digitally signed messages rather
     than depending on the integrity of the transport system.
4179
     Efforts to make it more difficult for users to set envelope return
     path and header "From" fields to point to valid addresses other than
     their own are largely misguided: they frustrate legitimate
     applications in which mail is sent by one user on behalf of another,
     in which error (or normal) replies should be directed to a specia


l
     address, or in which a single message is sent to multiple recipients
     on different hosts.  (Systems that provide convenient ways for users
     to alter these header fields on a per-message basis should attempt to
     establish a primary and permanent mailbox address for the user so
     that Sender header fields within the message data can be generated
     sensibly.)
4191
4192
4193
4194
4195

4197












































  Internet-Draft        RFC5321-Numbered               Dec 2008
4199
4200
     This specification does not further address the authentication issues
     associated with SMTP other than to advocate that useful functionality
     not be disabled in the hope of providing some small margin of
     protection against a user who is trying to fake mail.
4205
  7.2.  "Blind" Copies
4207
     Addresses that do not appear in the message header section may appear
     in the RCPT commands to an SMTP server for a number of reasons.  The
     two most common involve the use of a mailing address as a "list
     exploder" (a single address that resolves into multiple addresses)
     and the appearance of "blind copies".  Especially when more than one
     RCPT command is present, and in order to avoid defeating some of the
     purpose of these mechanisms, SMTP clients and servers SHOULD NOT copy
     the full set of RCPT command arguments into the header section,
     either as part of trace header fields or as informational or private-
     extension header fields.  Since this rule is often violated in
     practice, and cannot be enforced, sending SMTP systems that are aware
     of "bcc" use MAY find it helpful to send each blind copy as a
     separate message transaction containing only a single RCPT command.
4221
     There is no inherent relationship between either "reverse" (from
     MAIL, SAML, etc., commands) or "forward" (RCPT) addresses in the SMTP
     transaction ("envelope") and the addresses in the header section.
     Receiving systems SHOULD NOT attempt to deduce such relationshipsand
     use them to alter the header section of the message for delivery.
     The popular "Apparently-to" header field is a violation of this
     principle as well as a common source of unintended information
     disclosure and SHOULD NOT be used.
4230
  7.3.  VRFY, EXPN, and Security
4232
     As discussed in Section 3.5, individual sites may want to disable
     either or both of VRFY or EXPN for security reasons (see below). As
     a corollary to the above, implementations that permit this MUST NOT
     appear to have verified addresses that are not, in fact, verified.
     If a site disables these commands for security reasons, the SMTP
     server MUST return a 252 response, rather than a code that could be


     confused with successful or unsuccessful verification.
4240
     Returning a 250 reply code with the address listed in the VRFY
     command after having checked it only for syntax violates this rule.
     Of course, an implementation that "supports" VRFY by always returning
     550 whether or not the address is valid is equally not in
     conformance.
4246
     On the public Internet, the contents of mailing lists have become
     popular as an address information source for so-called "spammers."
4249
4250
4251

4253












































  Internet-Draft        RFC5321-Numbered               Dec 2008
4255
4256
     The use of EXPN to "harvest" addresses has increased as list
     administrators have installed protections against inappropriate uses
     of the lists themselves.  However, VRFY and EXPN are still usefulfor
     authenticated users and within an administrative domain.  For
     example, VRFY and EXPN are useful for performing internal audits of
     how email gets routed to check and to make sure no one is
     automatically forwarding sensitive mail outside the organization.
     Sites implementing SMTP authentication may choose to make VRFY and
     EXPN available only to authenticated requestors.  Implementations
     SHOULD still provide support for EXPN, but sites SHOULD carefully
     evaluate the tradeoffs.
4268
     Whether disabling VRFY provides any real marginal security depends on
     a series of other conditions.  In many cases, RCPT commands can be
     used to obtain the same information about address validity.  On the
     other hand, especially in situations where determination of address
     validity for RCPT commands is deferred until after the DATA command
     is received, RCPT may return no information at all, while VRFY is
     expected to make a serious attempt to determine validity before
     generating a response code (see discussion above).
4277
  7.4.  Mail Rerouting Based on the 251 and 551 Response Codes
4279
     Before a client uses the 251 or 551 reply codes from a RCPT command
     to automatically update its future behavior (e.g., updating the
     user's address book), it should be certain of the server's
     authenticity.  If it does not, it may be subject to a man in the
     middle attack.
4285
  7.5.  Information Disclosure in Announcements
4287
     There has been an ongoing debate about the tradeoffs between the
     debugging advantages of announcing server type and version (and,
     sometimes, even server domain name) in the greeting response or in
     response to the HELP command and the disadvantages of exposing
     information that might be useful in a potential hostile attack.  The
     utility of the debugging information is beyond doubt.  Those who
     argue for making it available point out that it is far better to
     actually secure an SMTP server rather than hope that trying to
     conceal known vulnerabilities by hiding the server's precise identity
     will provide more protection.  Sites are encouraged to evaluate the


     tradeoff with that issue in mind; implementations SHOULD minimally
     provide for making type and version information available in someway
     to other network hosts.
4301
4302
4303
4304
4305
4306
4307

4309
















































  Internet-Draft        RFC5321-Numbered               Dec 2008
4311
4312
  7.6.  Information Disclosure in Trace Fields
4314
     In some circumstances, such as when mail originates from within aLAN
     whose hosts are not directly on the public Internet, trace
     ("Received") header fields produced in conformance with this
     specification may disclose host names and similar information that
     would not normally be available.  This ordinarily does not pose a
     problem, but sites with special concerns about name disclosure should
     be aware of it.  Also, the optional FOR clause should be supplied
     with caution or not at all when multiple recipients are involved lest
     it inadvertently disclose the identities of "blind copy" recipients
     to others.
4325
  7.7.  Information Disclosure in Message Forwarding
4327
     As discussed in Section 3.4, use of the 251 or 551 reply codes to
     identify the replacement address associated with a mailbox may
     inadvertently disclose sensitive information.  Sites that are
     concerned about those issues should ensure that they select and
     configure servers appropriately.
4333
  7.8.  Resistance to Attacks
4335
     In recent years, there has been an increase of attacks on SMTP
     servers, either in conjunction with attempts to discover addresses
     for sending unsolicited messages or simply to make the servers
     inaccessible to others (i.e., as an application-level denial of
     service attack).  While the means of doing so are beyond the scope of
     this Standard, rational operational behavior requires that servers be
     permitted to detect such attacks and take action to defend
     themselves.  For example, if a server determines that a large number
     of RCPT TO commands are being sent, most or all with invalid
     addresses, as part of such an attack, it would be reasonable for the
     server to close the connection after generating an appropriate number
     of 5yz (normally 550) replies.
4348
  7.9.  Scope of Operation of SMTP Servers
4350
     It is a well-established principle that an SMTP server may refuseto
     accept mail for any operational or technical reason that makes sense
     to the site providing the server.  However, cooperation among sites


     and installations makes the Internet possible.  If sites take
     excessive advantage of the right to reject traffic, the ubiquity of
     email availability (one of the strengths of the Internet) will be
     threatened; considerable care should be taken and balance maintained
     if a site decides to be selective about the traffic it will accept
     and process.
4360
4361
4362
4363

4365















































  Internet-Draft        RFC5321-Numbered               Dec 2008
4367
4368
     In recent years, use of the relay function through arbitrary sites
     has been used as part of hostile efforts to hide the actual origins
     of mail.  Some sites have decided to limit the use of the relay
     function to known or identifiable sources, and implementations SHOULD
     provide the capability to perform this type of filtering.  When mail
     is rejected for these or other policy reasons, a 550 code SHOULD be
     used in response to EHLO (or HELO), MAIL, or RCPT as appropriate.
4376
  8.  IANA Considerations
4378
     IANA maintains three registries in support of this specification,all
     of which were created for RFC 2821 or earlier.  This document expands
     the third one as specified below.  The registry references listedare
     as of the time of publication; IANA does not guarantee the locations
     associated with the URLs.  The registries are as follows:
4384
     o  The first, "Simple Mail Transfer Protocol (SMTP) Service
        Extensions" [46], consists of SMTP service extensions with the
        associated keywords, and, as needed, parameters and verbs.  As
        specified in Section 2.2.2, no entry may be made in this registry
        that starts in an "X".  Entries may be made only for service
        extensions (and associated keywords, parameters, or verbs) that
        are defined in Standards-Track or Experimental RFCs specifically
        approved by the IESG for this purpose.
4393
     o  The second registry, "Address Literal Tags" [47], consists of
        "tags" that identify forms of domain literals other than thosefor
        IPv4 addresses (specified in RFC 821 and in this document).  The
        initial entry in that registry is for IPv6 addresses (specified in
        this document).  Additional literal types require standardization
        before being used; none are anticipated at this time.
4400
     o  The third, "Mail Transmission Types" [46], established by RFC 821
        and renewed by this specification, is a registry of link and
        protocol identifiers to be used with the "via" and "with"
        subclauses of the time stamp ("Received:" header field) described
        in Section 4.4.  Link and protocol identifiers in addition to


        those specified in this document may be registered only by
        standardization or by way of an RFC-documented, IESG-approved,
        Experimental protocol extension.  This name space is for
        identification and not limited in size: the IESG is encouragedto
        approve on the basis of clear documentation and a distinct method
        rather than preferences about the properties of the method itself.
4412
        An additional subsection has been added to the "VIA link types"
        and "WITH protocol types" subsections of this registry to contain
        registrations of "Additional-registered-clauses" as described
        above.  The registry will contain clause names, a description,a
4417
4418
4419

4421








































  Internet-Draft        RFC5321-Numbered               Dec 2008
4423
4424
        summary of the syntax of the associated String, and a reference.
        As new clauses are defined, they may, in principle, specify
        creation of their own registries if the Strings consist of
        reserved terms or keywords rather than less restricted strings.
        As with link and protocol identifiers, additional clauses may be
        registered only by standardization or by way of an RFC-documented,
        IESG-approved, Experimental protocol extension.  The additional
        clause name space is for identification and is not limited in
        size: the IESG is encouraged to approve on the basis of clear
        documentation, actual use or strong signs that the clause willbe
        used, and a distinct requirement rather than preferences aboutthe
        properties of the clause itself.
4437
     In addition, if additional trace header fields (i.e., in additionto
     Return-path and Received) are ever created, those trace fields MUST
     be added to the IANA registry established by BCP 90 (RFC 3864) [11]
     for use with RFC 5322 [4].
4442
  9.  Acknowledgments
4444
     Many people contributed to the development of RFC 2821.  That
     document should be consulted for those acknowledgments.  For the
     present document, the editor and the community owe thanks to Dawn
     Mann and Tony Hansen who assisted in the very painful process of
     editing and converting the internal format of the document from one
     system to another.
4451
     Neither this document nor RFC 2821 would have been possible without
     the many contribution and insights of the late Jon Postel.  Those
     contributions of course include the original specification of SMTP in
     RFC 821.  A considerable quantity of text from RFC 821 still appears
     in this document as do several of Jon's original examples that have
     been updated only as needed to reflect other changes in the
     specification.
4459
     Many people made comments or suggestions on the mailing list or in
     notes to the author.  Important corrections or clarifications were
     suggested by several people, including Matti Aarnio, Glenn Anders


on,
     Derek J. Balling, Alex van den Bogaerdt, Stephane Bortzmeyer, Vint
     Cerf, Jutta Degener, Steve Dorner, Lisa Dusseault, Frank Ellerman,
     Ned Freed, Randy Gellens, Sabahattin Gucukoglu, Philip Guenther, Arnt
     Gulbrandsen, Eric Hall, Richard O. Hammer, Tony Hansen, Peter J.
     Holzer, Kari Hurtta, Bryon Roche Kain, Valdis Kletnieks, Mathias
     Koerber, John Leslie, Bruce Lilly, Jeff Macdonald, Mark E. Mallett,
     Mark Martinec, S. Moonesamy, Lyndon Nerenberg, Chris Newman, Douglas
     Otis, Pete Resnick, Robert A. Rosenberg, Vince Sabio, Hector Santos,
     David F. Skoll, Paul Smith, and Brett Watson.
4472
4473
4474
4475

4477








































  Internet-Draft        RFC5321-Numbered               Dec 2008
4479
4480
     The efforts of the Area Directors -- Lisa Dusseault, Ted Hardie, and
     Chris Newman -- to get this effort restarted and keep it moving, and
     of an ad hoc committee with the same purpose, are gratefully
     acknowledged.  The members of that committee were (in alphabetical
     order) Dave Crocker, Cyrus Daboo, Tony Finch, Ned Freed, Randall
     Gellens, Tony Hansen, the author, and Alexey Melnikov.  Tony Hansen
     also acted as ad hoc chair on the mailing list reviewing this
     document; without his efforts, sense of balance and fairness, and
     patience, it clearly would not have been possible.
4490
  10.  References
4492
  10.1.  Normative References
4494
     [1]   Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
           August 1982.
4497
     [2]   Mockapetris, P., "Domain names - implementation and
           specification", STD 13, RFC 1035, November 1987.
4500
     [3]   Braden, R., "Requirements for Internet Hosts - Application and
           Support", STD 3, RFC 1123, October 1989.
4503
     [4]   Resnick, P., "Internet Message Format", RFC 5322, October 2008.
4505
     [5]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
           Levels", BCP 14, RFC 2119, March 1997.
4508
     [6]   American National Standards Institute (formerly United States
           of America Standards Institute), "USA Code for Information
           Interchange", ANSI X3.4-1968, 1968.
4512
           ANSI X3.4-1968 has been replaced by newer versions with slight
           modifications, but the 1968 version remains definitive for the
           Internet.
4516
     [7]   Crocker, D. and P. Overell, "Augmented BNF for Syntax
           Specifications: ABNF", STD 68, RFC 5234, January 2008.
4519
     [8]   Hinden, R. and S. Deering, "IP Version 6 Addressing
           Architecture", RFC 4291, February 2006.
4522
     [9]   Newman, C., "ESMTP and LMTP Transmission Types Registration",


           RFC 3848, July 2004.
4525
     [10]  Klensin, J., Freed, N., and K. Moore, "SMTP Service Extension
           for Message Size Declaration", STD 10, RFC 1870, November 1995.
4528
4529
4530
4531

4533


















































  Internet-Draft        RFC5321-Numbered               Dec 2008
4535
4536
     [11]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
           Procedures for Message Header Fields", BCP 90, RFC 3864,
           September 2004.
4540
  10.2.  Informative References
4542
     [12]  Partridge, C., "Mail routing and the domain system", RFC 974,
           January 1986.
4545
     [13]  Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
           Crocker, "SMTP Service Extensions", STD 10, RFC 1869,
           November 1995.
4549
     [14]  Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
           April 2001.
4552
     [15]  Butler, M., Postel, J., Chase, D., Goldberger, J., and J.
           Reynolds, "Post Office Protocol: Version 2", RFC 937,
           February 1985.
4556
     [16]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
           STD 53, RFC 1939, May 1996.
4559
     [17]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
           4rev1", RFC 3501, March 2003.
4562
     [18]  Gellens, R. and J. Klensin, "Message Submission for Mail",
           RFC 4409, April 2006.
4565
     [19]  Freed, N., "SMTP Service Extension for Command Pipelining",
           STD 60, RFC 2920, September 2000.
4568
     [20]  Vaudreuil, G., "SMTP Service Extensions for Transmission of
           Large and Binary MIME Messages", RFC 3030, December 2000.
4571
     [21]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
           Extensions (MIME) Part One: Format of Internet Message Bodies",
           RFC 2045, November 1996.
4575
     [22]  Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
           Crocker, "SMTP Service Extension for 8bit-MIMEtransport",
           RFC 1652, July 1994.
4579
     [23]  Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
           Three: Message Header Extensions for Non-ASCII Text", RFC 2047,
           November 1996.
4583
4584
4585
4586
4587



4589




























































  Internet-Draft        RFC5321-Numbered               Dec 2008
4591
4592
     [24]  Freed, N. and K. Moore, "MIME Parameter Value and Encoded Word
           Extensions: Character Sets, Languages, and Continuations",
           RFC 2231, November 1997.
4596
     [25]  Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 3463,
           January 2003.
4599
     [26]  Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced Mail
           System Status Codes", BCP 138, RFC 5248, June 2008.
4602
     [27]  Freed, N., "Behavior of and Requirements for Internet
           Firewalls", RFC 2979, October 2000.
4605
     [28]  Crocker, D., "Standard for the format of ARPA Internet text
           messages", STD 11, RFC 822, August 1982.
4608
     [29]  Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) for
           Authorizing Use of Domains in E-Mail, Version 1", RFC 4408,
           April 2006.
4612
     [30]  Fenton, J., "Analysis of Threats Motivating DomainKeys
           Identified Mail (DKIM)", RFC 4686, September 2006.
4615
     [31]  Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J.,and
           M. Thomas, "DomainKeys Identified Mail (DKIM) Signatures",
           RFC 4871, May 2007.
4619
     [32]  Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
           Extension for Delivery Status Notifications (DSNs)", RFC 3461,
           January 2003.
4623
     [33]  Moore, K. and G. Vaudreuil, "An Extensible Message Format for
           Delivery Status Notifications", RFC 3464, January 2003.
4626
     [34]  Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9,
           RFC 959, October 1985.
4629
     [35]  Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay): Mapping
           between X.400 and RFC 822/MIME", RFC 2156, January 1998.
4632
     [36]  De Winter, J., "SMTP Service Extension for Remote Message Queue
           Starting", RFC 1985, August 1996.
4635
     [37]  Hansen, T. and G. Vaudreuil, "Message Disposition
           Notification", RFC 3798, May 2004.
4638


     [38]  Elz, R. and R. Bush, "Clarifications to the DNS Specification",
           RFC 2181, July 1997.
4641
4642
4643

4645






















































  Internet-Draft        RFC5321-Numbered               Dec 2008
4647
4648
     [39]  Nakamura, M. and J. Hagino, "SMTP Operational Experience in
           Mixed IPv4/v6 Environments", RFC 3974, January 2005.
4651
     [40]  Partridge, C., "Duplicate messages and SMTP", RFC 1047,
           February 1988.
4654
     [41]  Crispin, M., "Interactive Mail Access Protocol: Version 2",
           RFC 1176, August 1990.
4657
     [42]  Lambert, M., "PCMAIL: A distributed mail system for personal
           computers", RFC 1056, June 1988.
4660
     [43]  Galvin, J., Murphy, S., Crocker, S., and N. Freed, "Security
           Multiparts for MIME: Multipart/Signed and Multipart/Encrypted",
           RFC 1847, October 1995.
4664
     [44]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
           Thayer, "OpenPGP Message Format", RFC 4880, November 2007.
4667
     [45]  Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
           (S/MIME) Version 3.1 Message Specification", RFC 3851,
           July 2004.
4671
     [46]  Internet Assigned Number Authority (IANA), "IANA Mail
           Parameters", 2007,
           <http://www.iana.org/assignments/mail-parameters>.
4675
     [47]  Internet Assigned Number Authority (IANA), "Address Literal
           Tags", 2007,
           <http://www.iana.org/assignments/address-literal-tags>.
4679
4680
4681
4682
4683
4684
4685
4686
4687
4688
4689
4690
4691
4692
4693
4694
4695
4696
4697
4698
4699



84]
4701





























































  Internet-Draft        RFC5321-Numbered               Dec 2008
4703
4704
  Appendix A.  TCP Transport Service
4706
     The TCP connection supports the transmission of 8-bit bytes.  The
     SMTP data is 7-bit ASCII characters.  Each character is transmitted
     as an 8-bit byte with the high-order bit cleared to zero.  Service
     extensions may modify this rule to permit transmission of full 8-bit
     data bytes as part of the message body, or, if specifically designed
     to do so, in SMTP commands or responses.
4713
  Appendix B.  Generating SMTP Commands from RFC 822 Header Fields
4715
     Some systems use an RFC 822 header section (only) in a mail
     submission protocol, or otherwise generate SMTP commands from RFC822
     header fields when such a message is handed to an MTA from a UA.
     While the MTA-UA protocol is a private matter, not covered by any
     Internet Standard, there are problems with this approach.  For
     example, there have been repeated problems with proper handling of
     "bcc" copies and redistribution lists when information that
     conceptually belongs to the mail envelope is not separated early in
     processing from header field information (and kept separate).
4725
     It is recommended that the UA provide its initial ("submission
     client") MTA with an envelope separate from the message itself.
     However, if the envelope is not supplied, SMTP commands SHOULD be
     generated as follows:
4730
     1.  Each recipient address from a TO, CC, or BCC header field SHOULD
         be copied to a RCPT command (generating multiple message copies
         if that is required for queuing or delivery).  This includes any
         addresses listed in a RFC 822 "group".  Any BCC header fields
         SHOULD then be removed from the header section.  Once this
         process is completed, the remaining header fields SHOULD be
         checked to verify that at least one TO, CC, or BCC header field
         remains.  If none do, then a BCC header field with no additional
         information SHOULD be inserted as specified in [4].
4740
     2.  The return address in the MAIL command SHOULD, if possible, be
         derived from the system's identity for the submitting (local)
         user, and the "From:" header field otherwise.  If there is a
         system identity available, it SHOULD also be copied to the Sender
         header field if it is different from the address in the From


         header field.  (Any Sender header field that was already there
         SHOULD be removed.)  Systems may provide a way for submittersto
         override the envelope return address, but may want to restrict
         its use to privileged users.  This will not prevent mail forgery,
         but may lessen its incidence; see Section 7.1.
4751
4752
4753
4754
4755

4757














































  Internet-Draft        RFC5321-Numbered               Dec 2008
4759
4760
     When an MTA is being used in this way, it bears responsibility for
     ensuring that the message being transmitted is valid.  The mechanisms
     for checking that validity, and for handling (or returning) messages
     that are not valid at the time of arrival, are part of the MUA-MTA
     interface and not covered by this specification.
4766
     A submission protocol based on Standard RFC 822 information alone
     MUST NOT be used to gateway a message from a foreign (non-SMTP) mail
     system into an SMTP environment.  Additional information to construct
     an envelope must come from some source in the other environment,
     whether supplemental header fields or the foreign system's envelope.
4772
     Attempts to gateway messages using only their header "To" and "Cc"
     fields have repeatedly caused mail loops and other behavior adverse
     to the proper functioning of the Internet mail environment.  These
     problems have been especially common when the message originates from
     an Internet mailing list and is distributed into the foreign
     environment using envelope information.  When these messages are then
     processed by a header-section-only remailer, loops back to the
     Internet environment (and the mailing list) are almost inevitable.
4781
  Appendix C.  Source Routes
4783
     Historically, the <reverse-path> was a reverse source routing list of
     hosts and a source mailbox.  The first host in the <reverse-path>was
     historically the host sending the MAIL command; today, source routes
     SHOULD NOT appear in the reverse-path.  Similarly, the <forward-path>
     may be a source routing lists of hosts and a destination mailbox.
     However, in general, the <forward-path> SHOULD contain only a mailbox
     and domain name, relying on the domain name system to supply routing
     information if required.  The use of source routes is deprecated (see
     Appendix F.2); while servers MUST be prepared to receive and handle
     them as discussed in Section 3.3 and Appendix F.2, clients SHOULDNOT


     transmit them and this section is included in the current
     specification only to provide context.  It has been modified somewhat
     from the material in RFC 821 to prevent server actions that might
     confuse clients or subsequent servers that do not expect a full
     source route implementation.
4799
     For relay purposes, the forward-path may be a source route of the
     form "@ONE,@TWO:JOE@THREE", where ONE, TWO, and THREE MUST be fully-
     qualified domain names.  This form is used to emphasize the
     distinction between an address and a route.  The mailbox (here, JOE@
     THREE) is an absolute address, and the route is information abouthow
     to get there.  The two concepts should not be confused.
4806
     If source routes are used, RFC 821 and the text below should be
     consulted for the mechanisms for constructing and updating the
4809
4810
4811

4813






































  Internet-Draft        RFC5321-Numbered               Dec 2008
4815
4816
     forward-path.  A server that is reached by means of a source route
     (e.g., its domain name appears first in the list in the forward-path)
     MUST remove its domain name from any forward-paths in which that
     domain name appears before forwarding the message and MAY remove all
     other source routing information.  The reverse-path SHOULD NOT be
     updated by servers conforming to this specification.
4823
     Notice that the forward-path and reverse-path appear in the SMTP
     commands and replies, but not necessarily in the message.  That is,
     there is no need for these paths and especially this syntax to appear
     in the "To:" , "From:", "CC:", etc. fields of the message header
     section.  Conversely, SMTP servers MUST NOT derive final message
     routing information from message header fields.
4830
     When the list of hosts is present despite the recommendations above,
     it is a "reverse" source route and indicates that the mail was
     relayed through each host on the list (the first host in the listwas
     the most recent relay).  This list is used as a source route to
     return non-delivery notices to the sender.  If, contrary to the
     recommendations here, a relay host adds itself to the beginning of
     the list, it MUST use its name as known in the transport environment
     to which it is relaying the mail rather than that of the transport
     environment from which the mail came (if they are different).  Note
     that a situation could easily arise in which some relay hosts add
     their names to the reverse source route and others do not, generating
     discontinuities in the routing list.  This is another reason why
     servers needing to return a message SHOULD ignore the source route
     entirely and simply use the domain as specified in the Mailbox.
4845
  Appendix D.  Scenarios
4847
     This section presents complete scenarios of several types of SMTP
     sessions.  In the examples, "C:" indicates what is said by the SMTP
     client, and "S:" indicates what is said by the SMTP server.
4851
4852
4853
4854
4855
4856
4857


4858
4859
4860
4861
4862
4863
4864
4865
4866
4867

4869


















































  Internet-Draft        RFC5321-Numbered               Dec 2008
4871
4872
  D.1.  A Typical SMTP Transaction Scenario
4874
     This SMTP example shows mail sent by Smith at host bar.com, and to
     Jones, Green, and Brown at host foo.com.  Here we assume that host
     bar.com contacts host foo.com directly.  The mail is accepted for
     Jones and Brown.  Green does not have a mailbox at host foo.com.
4879
        S: 220 foo.com Simple Mail Transfer Service Ready
        C: EHLO bar.com
        S: 250-foo.com greets bar.com
        S: 250-8BITMIME
        S: 250-SIZE
        S: 250-DSN
        S: 250 HELP
        C: MAIL FROM:<Smith@bar.com>
        S: 250 OK
        C: RCPT TO:<Jones@foo.com>
        S: 250 OK
        C: RCPT TO:<Green@foo.com>
        S: 550 No such user here
        C: RCPT TO:<Brown@foo.com>
        S: 250 OK
        C: DATA
        S: 354 Start mail input; end with <CRLF>.<CRLF>
        C: Blah blah blah...
        C: ...etc. etc. etc.
        C: .
        S: 250 OK
        C: QUIT
        S: 221 foo.com Service closing transmission channel
4903
4904
4905
4906
4907
4908
4909
4910
4911
4912
4913
4914
4915
4916
4917
4918
4919
4920
4921
4922
4923



4925






























































  Internet-Draft        RFC5321-Numbered               Dec 2008
4927
4928
  D.2.  Aborted SMTP Transaction Scenario
4930
        S: 220 foo.com Simple Mail Transfer Service Ready
        C: EHLO bar.com
        S: 250-foo.com greets bar.com
        S: 250-8BITMIME
        S: 250-SIZE
        S: 250-DSN
        S: 250 HELP
        C: MAIL FROM:<Smith@bar.com>
        S: 250 OK
        C: RCPT TO:<Jones@foo.com>
        S: 250 OK
        C: RCPT TO:<Green@foo.com>
        S: 550 No such user here
        C: RSET
        S: 250 OK
        C: QUIT
        S: 221 foo.com Service closing transmission channel
4948
4949
4950
4951
4952
4953
4954
4955
4956
4957
4958
4959
4960
4961
4962
4963
4964
4965
4966
4967
4968
4969
4970
4971
4972
4973
4974
4975
4976
4977
4978
4979

4981




  Internet-Draft        RFC5321-Numbered               Dec 2008
4983
4984
  D.3.  Relayed Mail Scenario
4986
     Step 1 -- Source Host to Relay Host
4988
     The source host performs a DNS lookup on XYZ.COM (the destination
     address) and finds DNS MX records specifying xyz.com as the best
     preference and foo.com as a lower preference.  It attempts to open a
     connection to xyz.com and fails.  It then opens a connection to
     foo.com, with the following dialogue:
4994
        S: 220 foo.com Simple Mail Transfer Service Ready
        C: EHLO bar.com
        S: 250-foo.com greets bar.com
        S: 250-8BITMIME
        S: 250-SIZE
        S: 250-DSN
        S: 250 HELP
        C: MAIL FROM:<JQP@bar.com>
        S: 250 OK
        C: RCPT TO:<Jones@XYZ.COM>
        S: 250 OK
        C: DATA
        S: 354 Start mail input; end with <CRLF>.<CRLF>
        C: Date: Thu, 21 May 1998 05:33:29 -0700
        C: From: John Q. Public <JQP@bar.com>
        C: Subject: The Next Meeting of the Board
        C: To: Jones@xyz.com
        C:
        C: Bill:
        C: The next meeting of the board of directors will be
        C: on Tuesday.
        C: John.
        C: .
        S: 250 OK
        C: QUIT
        S: 221 foo.com Service closing transmission channel
5021
5022
5023
5024
5025
5026
5027
5028
5029
5030
5031
5032
5033
5034
5035

5037



  Internet-Draft        RFC5321-Numbered               Dec 2008
5039
5040
     Step 2 -- Relay Host to Destination Host
5042
     foo.com, having received the message, now does a DNS lookup on
     xyz.com.  It finds the same set of MX records, but cannot use theone
     that points to itself (or to any other host as a worse preference).
     It tries to open a connection to xyz.com itself and succeeds.  Then
     we have:
5048
             S: 220 xyz.com Simple Mail Transfer Service Ready
             C: EHLO foo.com
             S: 250 xyz.com is on the air
             C: MAIL FROM:<JQP@bar.com>
             S: 250 OK
             C: RCPT TO:<Jones@XYZ.COM>
             S: 250 OK
             C: DATA
             S: 354 Start mail input; end with <CRLF>.<CRLF>
             C: Received: from bar.com by foo.com ; Thu, 21 May 1998
             C:     05:33:29 -0700
             C: Date: Thu, 21 May 1998 05:33:22 -0700
             C: From: John Q. Public <JQP@bar.com>
             C: Subject:  The Next Meeting of the Board
             C: To: Jones@xyz.com
             C:
             C: Bill:
             C: The next meeting of the board of directors will be
             C: on Tuesday.
             C:                         John.
             C: .
             S: 250 OK
             C: QUIT
             S: 221 foo.com Service closing transmission channel
5073
5074
5075
5076
5077
5078
5079
5080
5081
5082
5083
5084
5085
5086
5087
5088
5089
5090
5091



91]
5093





























































  Internet-Draft        RFC5321-Numbered               Dec 2008
5095
5096
  D.4.  Verifying and Sending Scenario
5098
        S: 220 foo.com Simple Mail Transfer Service Ready
        C: EHLO bar.com
        S: 250-foo.com greets bar.com
        S: 250-8BITMIME
        S: 250-SIZE
        S: 250-DSN
        S: 250-VRFY
        S: 250 HELP
        C: VRFY Crispin
        S: 250 Mark Crispin <Admin.MRC@foo.com>
        C: MAIL FROM:<EAK@bar.com>
        S: 250 OK
        C: RCPT TO:<Admin.MRC@foo.com>
        S: 250 OK
        C: DATA
        S: 354 Start mail input; end with <CRLF>.<CRLF>
        C: Blah blah blah...
        C: ...etc. etc. etc.
        C: .
        S: 250 OK
        C: QUIT
        S: 221 foo.com Service closing transmission channel
5121
  Appendix E.  Other Gateway Issues
5123
     In general, gateways between the Internet and other mail systems
     SHOULD attempt to preserve any layering semantics across the
     boundaries between the two mail systems involved.  Gateway-
     translation approaches that attempt to take shortcuts by mapping
     (such as mapping envelope information from one system to the message
     header section or body of another) have generally proven to be
     inadequate in important ways.  Systems translating between
     environments that do not support both envelopes and a header section
     and Internet mail must be written with the understanding that some
     information loss is almost inevitable.
5134
5135
5136
5137
5138
5139
5140
5141
5142
5143
5144
5145
5146
5147



92]
5149





























































  Internet-Draft        RFC5321-Numbered               Dec 2008
5151
5152
  Appendix F.  Deprecated Features of RFC 821
5154
     A few features of RFC 821 have proven to be problematic and SHOULD
     NOT be used in Internet mail.
5157
  F.1.  TURN
5159
     This command, described in RFC 821, raises important security issues
     since, in the absence of strong authentication of the host requesting
     that the client and server switch roles, it can easily be used to
     divert mail from its correct destination.  Its use is deprecated;
     SMTP systems SHOULD NOT use it unless the server can authenticatethe
     client.
5166
  F.2.  Source Routing
5168
     RFC 821 utilized the concept of explicit source routing to get mail
     from one host to another via a series of relays.  The requirementto
     utilize source routes in regular mail traffic was eliminated by the
     introduction of the domain name system "MX" record and the last
     significant justification for them was eliminated by the
     introduction, in RFC 1123, of a clear requirement that addresses
     following an "@" must all be fully-qualified domain names.
     Consequently, the only remaining justifications for the use of source
     routes are support for very old SMTP clients or MUAs and in mail
     system debugging.  They can, however, still be useful in the latter
     circumstance and for routing mail around serious, but temporary,
     problems such as problems with the relevant DNS records.
5181
     SMTP servers MUST continue to accept source route syntax as specified
     in the main body of this document and in RFC 1123.  They MAY, if
     necessary, ignore the routes and utilize only the target domain in
     the address.  If they do utilize the source route, the message MUST
     be sent to the first domain shown in the address.  In particular,a
     server MUST NOT guess at shortcuts within the source route.
5188
     Clients SHOULD NOT utilize explicit source routing except under
     unusual circumstances, such as debugging or potentially relaying
     around firewall or mail system configuration errors.
5192
  F.3.  HELO
5194


     As discussed in Sections 3.1 and 4.1.1, EHLO SHOULD be used rather
     than HELO when the server will accept the former.  Servers MUST
     continue to accept and process HELO in order to support older
     clients.
5199
5200
5201
5202
5203

5205


















































  Internet-Draft        RFC5321-Numbered               Dec 2008
5207
5208
  F.4.  #-literals
5210
     RFC 821 provided for specifying an Internet address as a decimal
     integer host number prefixed by a pound sign, "#".  In practice, that
     form has been obsolete since the introduction of TCP/IP.  It is
     deprecated and MUST NOT be used.
5215
  F.5.  Dates and Years
5217
     When dates are inserted into messages by SMTP clients or servers
     (e.g., in trace header fields), four-digit years MUST BE used.  Two-
     digit years are deprecated; three-digit years were never permitted in
     the Internet mail system.
5222
  F.6.  Sending versus Mailing
5224
     In addition to specifying a mechanism for delivering messages to
     user's mailboxes, RFC 821 provided additional, optional, commandsto
     deliver messages directly to the user's terminal screen.  These
     commands (SEND, SAML, SOML) were rarely implemented, and changes in
     workstation technology and the introduction of other protocols may
     have rendered them obsolete even where they are implemented.
5231
     Clients SHOULD NOT provide SEND, SAML, or SOML as services.  Servers
     MAY implement them.  If they are implemented by servers, the
     implementation model specified in RFC 821 MUST be used and the
     command names MUST be published in the response to the EHLO command.

Author's Address

   John C. Klensin
   1770 Massachusetts Ave, Suite 322
   Cambridge, MA  02140
   USA

   EMail: john+smtp@jck.com

5247
5248
5249
5250
5251
5252
5253
5254
5255


5256
5257
5258
5259

5261
























































  Internet-Draft        RFC5321-Numbered               Dec 2008

Full Copyright Statement

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   contained in BCP 78, and except as set forth therein, the authors
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Klensin                     Standards Track                    [Page 95]