INTERNET-DRAFT John C. Klensin, Editor
Expires December 2000
July 10, 2000
Simple Mail Transfer Protocol
draft-ietf-drums-smtpupd-12.txt
Status of this Memo
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[[Appendix X will be removed before the document is submitted to the
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[[If consensus is reached on this document, it will be forwarded to the
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Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Table of Contents
0. Abstract
1. Introduction
2. The SMTP Model
2.1 Basic Structure
2.2 The Extension Model
2.2.1 Background
2.2.2 Definition and Registration of Extensions
2.3 Terminology
2.3.1 Mail Objects
2.3.2 Senders and Receivers
2.3.3 Mail Agents and Message Stores
2.3.4 Host
2.3.5 Domain
2.3.6 Buffer and State Table
2.3.7 Lines
2.3.8 Originator, Delivery, Relay, and Gateway Systems
2.3.9 Message Content and Mail Data
2.3.10 Mailbox and Address
2.3.11 Reply
2.4 General Syntax Principles and Transaction Model
3. The SMTP Procedures: An Overview
3.1 Session Initiation
3.2 Client Initiation
3.3 Mail Transactions
3.4 Forwarding for Address Correction or Updating
3.5 Commands for Debugging Addresses
3.5.1 Overview
3.5.2 VRFY Normal Response
3.5.3 Meaning of VRFY or EXPN Success Response
3.5.4 Semantics and Applications of EXPN
3.6 Domains
3.7 Relaying
3.8 Mail Gatewaying
3.8.1 Header Fields in Gatewaying
3.8.2 Received Lines in Gatewaying
3.8.3 Addresses in Gatewaying
3.8.4 Other Header Fields in Gatewaying
3.8.5 Envelopes in Gatewaying
3.9 Terminating Sessions and Connections
3.10 Mailing Lists and Aliases
3.10.1 Alias
3.10.2 List
4. The SMTP Specifications
4.1 SMTP Commands
4.1.1 Command Semantics and Syntax
4.1.1.1 Extended HELLO (EHLO) or HELLO (HELO)
4.1.1.2 MAIL (MAIL)
4.1.1.3 RECIPIENT (RCPT)
4.1.1.4 DATA (DATA)
4.1.1.5 RESET (RSET)
4.1.1.6 VERIFY (VRFY)
4.1.1.7 EXPAND (EXPN)
4.1.1.8 HELP (HELP)
4.1.1.9 NOOP (NOOP)
4.1.1.10 QUIT (QUIT)
4.1.2 Command Argument Syntax
4.1.3 Address Literals
4.1.4 Order of Commands
4.1.5 Private-use Commands
4.2 SMTP Replies
4.2.1 Reply Code Severities and Theory
4.2.2 Reply Codes by Function Groups
4.2.3 Reply Codes in Numeric Order
4.2.4 Reply Code 502
4.2.5 Reply Codes After DATA and the Subsequent <CRLF>.<CRLF>
4.3 Sequencing of Commands and Replies
4.3.1 Sequencing Overview
4.3.2 Command-Reply Sequences
4.4 Trace Information
4.5 Additional Implementation Issues
4.5.1 Minimum Implementation
4.5.2 Transparency
4.5.3 Sizes and Timeouts
4.5.3.1 Size limits and minimums
4.5.3.2 Timeouts
4.5.4 Retry Strategies
4.5.4.1 Sending Strategy
4.5.4.2 Receiving Strategy
4.5.5 Messages with a null reverse-path
5. Address Resolution and Mail Handling
6. Problem Detection and Handling
6.1 Reliable Delivery and Replies by Email
6.2 Loop Detection
6.3 Compensating for Irregularities
7. Security Considerations
7.1 Mail Security and Spoofing
7.2 "Blind" Copies
7.3 VRFY, EXPN, and Security
7.4 Information Disclosure in Announcements
7.5 Information Disclosure in Trace Fields
7.6 Information Disclosure in Message Forwarding
7.7 Scope of Operation of SMTP Servers
8. IANA Considerations
9. References
10. Editors' Addresses
11. Acknowledgments
Appendices
A. TCP Transport Service
B. Generating SMTP Commands from RFC 822 Headers
C. Source Routes
D. Scenarios
E. Other Gateway Issues
F. Deprecated Features of RFC 821
X. Change Summary and Loose Ends (Temporary)
0. Abstract
This document is a self-contained specification of the basic protocol for
the Internet electronic mail transport, consolidating and updating:
- the original SMTP specification of RFC 821 [RFC-821],
- domain name system requirements and implications for mail transport from
RFC 1035 [RFC-DNS] and RFC 974 [RFC-974],
- the clarifications and applicability statements in RFC 1123 [RFC-1123],
and
- material drawn from the SMTP Extension mechanisms [SMTPEXT].
It replaces RFC 821, RFC 974, and 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.
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 turned up 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.
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, as recommended for POP [RFC-POP2, RFC-POP3]
and IMAP [RFC-IMAP4]. Additional submission issues are discussed in RFC
2476 [SUBMIT].
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.
A companion document [MSGFMT] discusses message headers, message bodies
and formats and structures for them, and their relationship.
Comments on this draft should be addressed to the IETF DRUMS Working
Group:
General Discussion:drums@cs.utk.edu
To Subscribe: drums-request@cs.utk.edu
Archive: ftp://cs.utk.edu/pub/drums/mail-archive/
1. Introduction
The objective of the Simple Mail Transfer Protocol (SMTP) is to transfer
mail reliably and efficiently.
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 describe some of them.
An important feature of SMTP is its capability to transport mail across
networks, usually referred to as "SMTP mail relaying" (see section 3.8).
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.
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-DNS, and
section 5 of this document] are used to identify the appropriate next-hop
destination for a message being transported.
2. The SMTP Model
2.1 Basic Structure
The SMTP design can be pictured as:
+----------+ +----------+
+------+ | | | |
| User |<-->| | SMTP | |
+------+ | Client- |Commands/Replies| Server- |
+------+ | SMTP |<-------------->| SMTP | +------+
| File |<-->| | and Mail | |<-->| File |
|System| | | | | |System|
+------+ +----------+ +----------+ +------+
SMTP client SMTP server
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.
The means by which a mail message is presented to an SMTP client, and how
that client determines the domain name(s) to which mail messages are to be
transferred is a local matter, and is not addressed by this document. In
some cases, the domain name(s) transferred to, or 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-POP2, RFC-POP3] or IMAP [RFC-IMAP4] protocols, or when the SMTP client
is inside an isolated transport service environment, the domain name
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 domain names 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.
The means by which an SMTP client, once it has determined a target domain
name, determines the identity of an SMTP server to which a copy of 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 channel to 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.
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.
In other words, message transfer can occur in a single connection between
the original SMTP-sender and the final SMTP-recipient, or can occur in a
series of hops through intermediary systems. In either case, a formal
handoff of responsibility for the message occurs: the protocol requires
that a server accept responsibility for either delivering a message or
properly reporting the failure to do so.
Once the transmission channel is established and initial handshaking
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 headers 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.
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 extension
requests such as in [RFC-Pipeline].
Once a given mail message has been transmitted, the client may either
request that the connection be shut down or may initiate other mail
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.
As suggested above, this protocol provides mechanisms for the transmission
of mail. This transmission normally occurs 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.
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.
Usually, intermediate hosts are determined via the DNS MX record, not by
explicit "source" routing (see section 5 and appendices C and F.2).
2.2 The Extension Model
2.2.1 Background
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.
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.
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 to be added, it must be done in
a way that permits older implementations to continue working acceptably.
The extension framework consists of:
- The SMTP command EHLO, superseding the earlier HELO,
- a registry of SMTP service extensions,
- additional parameters to the SMTP MAIL and RCPT commands, and
- optional replacements for commands defined in this protocol, such as for
DATA (see [RFC-BDAT]).
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.
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 extending the SMTP
service will likely outweigh the benefit.
2.2.2 Definition and Registration of Extensions
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:
- the textual name of the SMTP service extension;
- the EHLO keyword value associated with the extension;
- the syntax and possible values of parameters associated with the
EHLO keyword value;
- 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);
- any new parameters the extension associates with the MAIL or RCPT
verbs;
- a description of how support for the extension affects the behavior
of a server and client SMTP; and,
- the increment by which the extension is increasing the maximum
length of the commands MAIL and/or RCPT, over that specified
in this standard.
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.
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.
2.3 Terminology
Most of the terminology in this document is common in the Internet at the
time of its writing. However, the following terms and concepts are used
in special ways here, or represent differences in terminology between RFC
821 and this document, and should be understood before reading further.
These definitions are normative, that is, they contain specifications to
which SMTP implementations are required to conform.
The terms "MUST" and "SHOULD" (and "MUST NOT" and "SHOULD NOT") are
used in the same general sense here as in the Host Requirements
Standards [RFC-1123]. Specifically, "MUST" or "MUST NOT" identify
absolute requirements for conformance to this specification.
Implementations that do not conform to them lie outside the scope of
this specification and often will not interoperate properly with SMTP
implementations that do conform. Implementations that are fully
conforming also adhere to all "SHOULD" and "SHOULD NOT" requirements.
Implementations that adhere to all "MUST" ("MUST NOT") but not to all
of these are considered to be partially conforming. Such
implementations may interoperate properly with fully conforming ones
and with each other, but this will typically be the case only if great
care is taken. Consequently, an implementation should violate "SHOULD"
("SHOULD NOT") requirements only under exceptional and well-understood
circumstances. "SHOULD" (and sometimes "MUST") requirements are often
imposed by this specification when experience has shown that following
such requirements or restrictions leads, in practice, to better
interoperation, or smoother operation of the Internet email
infrastructure. As a consequence, some of these statements constitute
recommended practices, rather than the statistically most common
practice at the time of this writing. Statements using "MAY" describe
features or styles of doing things that may be followed, or not, at the
discretion of the implementation, normally without causing significant
interoperability problems.
2.3.1 Mail Objects
SMTP transports a mail object. A mail object contains an envelope and
content.
The SMTP envelope is sent as a series of SMTP protocol units (described
in section 3). It consists of an originator address (to which error
reports should be directed); one or more recipient addresses; and optional
protocol extension material. Historically, variations on the recipient
address specification command (RCPT TO) could be used to specify alternate
delivery modes, such as immediate display; those variations have now been
deprecated (see appendix F, section F.6).
The SMTP content is sent in the SMTP DATA protocol unit and has two
parts: the headers and the body. If the content conforms to other
contemporary standards, the headers form a collection of field/value
pairs structured as described in [MSGFMT]; the body, if structured, is
defined according to MIME [RFC-MIME]. The content is textual in nature,
expressed using the US-ASCII repertoire [US-ASCII]. Although SMTP
extensions (such as [8BitMIME]) may relax this restriction for the
content body, the content headers are always encoded using the US-ASCII
repertoire. The algorithm defined in [RFC-INTLHDR] is used to represent
header values outside the US-ASCII repertoire, while still encoding
them using the US-ASCII repertoire.
2.3.2 Senders and Receivers
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.
2.3.3 Mail Agents and Message Stores
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 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.
2.3.4 Host
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 "domain"); identifying them by numerical address is discouraged.
2.3.5 Domain
A domain (or domain name) consists of one or more dot-separated
components. These components ("labels" in DNS terminology [RFC-DNS]
are restricted for SMTP purposes to consist of a sequence of letters,
digits, and hyphens drawn from the ASCII character set [US-ASCII].
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 of a CNAME RR) or the label of Mail eXchanger records to be used
to deliver mail instead of representing a host name. See [RFC-DNS] and
section 5.
The domain name, as described in this document and in [RFC-DNS], 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.
2.3.6 Buffer and State Table
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 which may be
used by the client to, for example, "clear the buffer" or "reset the
state table," causing the information in the buffer to be discarded and
the state to be returned to some previous state.
2.3.7 Lines
SMTP commands and, unless altered by a service extension, message data,
are transmitted in "lines". Lines consist of zero or more data
characters terminated by the sequence ASCII character "CR" (hex value
0D) followed immediately by 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 4.5.3).
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.
2.3.8 Originator, Delivery, Relay, and Gateway Systems
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 which a mail user
agent is expected to subsequently access. A "relay" SMTP system
(usually referred to just as a "relay") receives mail from an 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.
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 [IAB-Firewalls].)
2.3.9 Message Content and Mail Data
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 message headers and the
possibly-structured message body. The MIME specification [RFC-MIME]
provides the standard mechanisms for structured message bodies.
2.3.10 Mailbox and Address
As used in this specification, an "address" is a character string that
identifies a user to whom mail will be sent or a location into 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 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.
2.3.11 Reply
An SMTP reply is an acknowledgment (positive or negative) sent 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. Recent work [RFC-Reply] has specified further structuring
of the reply strings, including the use of supplemental and more
specific completion codes.
2.4 General Syntax Principles and Transaction Model
SMTP commands and replies have a rigid syntax. All commands begin with
a four letter command verb. All Replies begin with a three digit
numeric code. In some commands and replies, arguments MUST follow 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.
Verbs and argument values (e.g., "TO:" or "to:" in the MAIL 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). That is, 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. This is NOT
true of a mailbox local-part. 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. Mailbox domains are
not case sensitive. 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.
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.
The argument field 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.
The syntax for each command is shown with the discussion of that
command. Common elements and parameters are shown in section 4.1.2.
Commands and replies are composed of characters from the ASCII
character set [US-ASCII]. 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 seven bit
transport only. An originating SMTP client which has not successfully
negotiated an appropriate extension with a particular server 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
mislabeled and the data path cannot accept the actual content, this may
result in ultimate delivery of a severely garbled message to the
recipient. Delivery SMTP systems MAY reject ("bounce") such messages
rather than deliver them. No sending SMTP system is 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.
Eight-bit message content transmission MAY be requested of the server
by a client using extended SMTP facilities, notably the "8BITMIME"
extension [8BITMIME]. 8BITMIME SHOULD be supported by SMTP servers.
However, it MUST not be construed as authorization to transmit
unrestricted eight bit material. 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.
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 [ABNF].
Metalanguage terms used in running text are surrounded by pointed
brackets (e.g., <CRLF>) for clarity.
3. The SMTP Procedures: An Overview
This section contains descriptions of the procedures used in SMTP:
session initiation, the mail transaction, forwarding mail, verifying
mailbox names and expanding mailing lists, and the 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.
3.1 Session Initiation
An SMTP session is initiated when a client opens a connection to a
server and the server responds with an opening message.
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 4.5.1).
The SMTP protocol allows a server to formally reject a transaction
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.
3.2 Client Initiation
Once the server has sent the 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 of the
extensions it supports. Older SMTP systems which are unable to support
service extensions and contemporary clients which 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.
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").
3.3 Mail Transactions
There are three steps to SMTP mail transactions. The transaction
starts with a MAIL command which gives the sender identification. 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.
The first step in the procedure is the MAIL command.
MAIL FROM:<reverse-path> [SP <mail-parameters> ] <CRLF>
This command tells the SMTP-receiver that a new mail transaction is
starting and to reset all its state tables and buffers, including any
recipients or mail data. The <reverse-path> portion of the first or
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 acceptable for 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 one or 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.
Historically, the <reverse-path> can contain more than just a mailbox,
however, contemporary systems SHOULD NOT use source routing (see
appendix C).
The optional <mail-parameters> are associated with negotiated SMTP
service extensions (see section 2.2).
The second step in the procedure is the RCPT command.
RCPT TO:<forward-path> [ SP <rcpt-parameters> ] <CRLF>
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 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). This
step of the procedure can be repeated any number of times.
The <forward-path> can contain more than just a mailbox. Historically,
the <forward-path> can be 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 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).
The third step in the procedure is the DATA command (or some
alternative specified in a service extension).
DATA <CRLF>
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 text is
successfully received and stored the SMTP-receiver sends a 250 OK reply.
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).
The end of mail data indicator also confirms the mail transaction and
tells the SMTP server to now process the stored recipients and mail
data. If accepted, the SMTP server returns a 250 OK reply. The DATA
command can fail at only two points in the protocol exchange:
- 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.
- 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), or 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.
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. 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.
When RFC 822 format is being used, the mail data include the memo
header items such as Date, Subject, To, Cc, From [MSGFMT]. Server SMTP
systems SHOULD NOT reject messages based on perceived defects in the
RFC 822 or MIME [RFC-MIME] message header or message body. In
particular, they MUST NOT reject messages in which the numbers of
Resent- fields do not match or Resent-to appears without Resent-from
and/or Resent-date.
Mail transaction commands MUST be used in the order discussed above.
3.4 Forwarding for Address Correction or Updating
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 current one.
Silent forwarding of messages (without server notification to the sender),
for security or non-disclosure purposes, is common in the contemporary
Internet.
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-effect of the forwarding
activity. This may be especially important when the 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.
In particular:
* 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. But,
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.
Alternately,
* Servers MAY reject or bounce messages when they are not deliverable when
addressed. When they do so, 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. But, 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.
SMTP server implementations that support the 251 and/or 551 reply codes are
strongly encouraged to provide configuration mechanisms so that sites which
conclude that they would undesirably disclose information can disable or
restrict their use.
3.5 Commands for Debugging Addresses
3.5.1 Overview
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 7.3).
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:
User Name <local-part@domain>
local-part@domain
When a name that is the argument to VRFY could identify more than one
mailbox, the server MAY either note the ambiguity or identify the
alternatives. In other words, any of the following are legitimate
response to VRFY:
553 User ambiguous
or
553- Ambiguous; Possibilities are
553-Joe Smith <jsmith@foo.com>
553-Harry Smith <hsmith@foo.com>
553 Melvin Smith <dweep@foo.com>
or
553-Ambiguous; Possibilities
553- <jsmith@foo.com>
553- <hsmith@foo.com>
553 <dweep@foo.com>
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-REPLY], will facilitate automated translation into other languages
as needed. Of course, a client that was highly automated or that was
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.
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.
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 of one
mailbox. If a request is made to verify 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 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").
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.
"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".
The case of expanding a mailbox list requires a multiline reply, such
as:
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>
or
C: EXPN Executive-Washroom-List
S: 550 Access Denied to You.
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.
3.5.2 VRFY Normal Response
When normal (2yz or 551) responses are returned from a VRFY or EXPN
request, the reply normally includes the mailbox name, i.e.,
"<local-part@domain>", where "domain" is a fully qualified domain name,
MUST appear in the syntax. 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 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.
Server implementations SHOULD support both VRFY and EXPN. For security
reasons, implementations MAY provide local installations a way to
disable either or both of these commands through configuration options
or the equivalent. When these commands are supported, they are not
required to work across relays when relaying is supported. Since they
were both optional in RFC 821, they MUST be listed as service
extensions in an EHLO response, if they are supported.
3.5.3 Meaning of VRFY or EXPN Success Response
A server MUST NOT return a 220 code in response to a VRFY or EXPN
command unless it has actually verified the address. In particular, a
server MUST NOT return 220 if all it has done is to verify that the
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.
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 least 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 discussed 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 that are recognized but that would be forwarded
or bounced 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.
3.5.4 Semantics and Applications of EXPN
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, and mail systems SHOULD NOT attempt them.
3.6 Domains
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 A RRs (as discussed in section 5) are permitted,
as are CNAME RRs whose targets can be resolved, in turn, to MX or A
RRs. Local nicknames or unqualified names MUST NOT be used. There are
two exceptions to the rule requiring FQDNs:
- The domain name given in the EHLO command MUST BE either a primary
host name (a domain name that resolves to an A RR) or, if the host
has no name, an address literal as described in section 4.1.1.1.
- 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.
3.7 Relaying
In general, the availability of Mail eXchanger records in the domain
name system [RFC-DNS, RFC-974] makes the use of explicit source routes
in the Internet mail system unnecessary. Many historical problems with
their interpretation 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 are also permitted to ignore the route
information and simply send to the final destination specified as the
last element in the route and SHOULD do so. There has been an invalid
practice of using names that do not appear in the DNS as destination
names, with the senders counting on 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.
When source routes are not used, the process described in RFC 821 for
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.
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 relay mail to a
particular address for policy reasons, a 550 response SHOULD be
returned.
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, and work is underway on standardized mail submission protocols
that might eventually supercede the current practices. In any event,
because these arrangements are private and fall outside the scope of
this specification, they are not described here.
It is important to note that MX records can point to SMTP servers which
act as gateways into other environments, not just SMTP relays and final
delivery systems; see sections 3.8 and 5.
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-NOTARY1]) SHOULD be used if possible.
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 section 4.5.5 for
additional discussion). A MAIL command with a null reverse-path
appears as follows:
MAIL FROM:<>
As discussed in section 2.4.1, a relay SMTP has no need to inspect or
act upon the headers or body of the message data and MUST NOT do so
except to add its own "Received:" header (section 4.4) and, optionally,
to attempt to detect looping in the mail system (see section 6.2).
3.8 Mail Gatewaying
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 2.3.8, when such a system is at the boundary
between two transport service environments, we refer to it as a
"gateway" or "gateway SMTP".
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 may be given for a
gateway between the Internet and another mail environment.
3.8.1 Header Fields in Gatewaying
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 2.4.1
Other mail systems gatewayed to the Internet often use a subset of
RFC-822 headers 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. 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).
3.8.2 Received Lines in Gatewaying
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.
"Received:" 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 fields arising in
non-SMTP environments, receiving systems MUST NOT reject mail based on
the format of a trace field and SHOULD be extremely robust in the light
of unexpected information or formats in those fields.
The gateway SHOULD indicate the environment and protocol in the "via"
clauses of Received field(s) that it supplies.
3.8.3 Addresses in Gatewaying
>From the Internet side, the gateway SHOULD accept all valid address
formats in SMTP commands and in RFC-822 headers, and all valid RFC-822
messages. Addresses and headers generated by gateways MUST conform to
applicable Internet standards (including this one and RFC-822).
Gateways are, of course, subject to the same rules for handling source
routes as those described for other SMTP systems in section 3.3.
3.8.4 Other Header Fields in Gatewaying
The gateway MUST ensure that all header fields of a message that it
forwards into the Internet mail environment meet the requirements for
Internet mail. In particular, all addresses in "From:", "To:",
"Cc:", etc., fields MUST be transformed (if necessary) to satisfy
RFC-822 syntax, MUST reference only fully-qualified domain names, and
MUST be effective and useful for sending replies. The translation
algorithm used to convert mail from the Internet protocols to another
environment's protocol SHOULD ensure that error messages from the
foreign mail environment are delivered to the return path from the
SMTP envelope, not to the sender listed in the "From:" field (or
other fields) of the RFC-822 message.
3.8.5 Envelopes in Gatewaying
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.
3.9 Terminating Sessions and Connections
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.
An SMTP server MUST NOT intentionally close the connection except:
- After receiving a QUIT command and responding with a 221 reply.
- After detecting the need to shutdown 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).
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.
An SMTP server which 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.
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 received and
act accordingly.
3.10 Mailing Lists and Aliases
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 (see [MSGFMT]) MUST be left unchanged;
in particular, the "From" field of the message header is unaffected.
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 that of the originator, is
strongly discouraged. We classify such a pseudo-mailbox as an "alias"
or a "list", depending upon the expansion rules.
3.10.1 Alias
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.
3.10.2 List
A mailing list may be said to operate by "redistribution" rather than
by "forwarding". To expand a list, the recipient mailer replaces the
pseudo-mailbox address in the envelope with all of the expanded
addresses. The return 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.
4. The SMTP Specifications
4.1 SMTP Commands
4.1.1 Command Semantics and Syntax
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 are encouraged to
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.
A mail transaction involves several data objects which 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 which
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.
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.
4.1.1.1 Extended HELLO (EHLO) or HELLO (HELO)
These commands are used to identify the SMTP client to the SMTP server.
The argument field 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 available),
the client SHOULD send an address literal (see section 4.1.3),
optionally followed by information that will help to identify the
client system.
The SMTP server identifies itself to the SMTP client in the connection
greeting reply and in the response to this command.
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.
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 tables and buffers
are cleared.
Syntax:
ehlo = "EHLO" SP Domain CRLF
helo = "HELO" SP Domain CRLF
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
keyworks 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 for a
positive response, using the ABNF notation and terminal symbols of
[ABNF], is:
ehlo-ok-rsp = ( "250" domain [ SP ehlo-greet ] CRLF )
/ ( "250-" domain [ SP ehlo-greet ] CRLF
*( "250-" ehlo-line CRLF )
"250" SP ehlo-line CRLF )
ehlo-greet = 1*(%d0-9 / %d11-12 / %d14-127)
; string of any characters other than CR or LF
ehlo-line = ehlo-keyword *( SP ehlo-param )
ehlo-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
; additional syntax of ehlo-params depends on
; ehlo-keyword
ehlo-param = 1*(%d33-127)
; any CHAR excluding <SP> and all
; control characters (US-ASCII 0-31 inclusive)
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 2.4.1.
4.1.1.2 MAIL (MAIL)
This command is used to initiate a mail transaction in which the mail
data is delivered to an SMTP server which may, in turn, deliver it to
one or more mailboxes or pass it on to another system (possibly using
SMTP). The argument field 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.
The reverse-path consists of the sender mailbox. Historically, that
mailbox might optionally have been preceeded 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 nondelivery notifications), the reverse-path may be null
(see section 3.7).
This command clears the reverse-path buffer, the forward-path buffer,
and the mail data buffer; and inserts the reverse-path information from
this command into the reverse-path buffer.
If service extensions were negotiated, the MAIL command may also carry
parameters associated with a particular service extension.
Syntax:
"MAIL FROM:" ("<>" / Reverse-Path)
[SP Mail-parameters] CRLF
4.1.1.3 RECIPIENT (RCPT)
This command is used to identify an individual recipient of the mail
data; multiple recipients are specified by multiple use of this
command. The argument field contains a forward-path and may contain
optional parameters.
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.
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.
For example, mail received at relay host xyz.com with envelope commands
MAIL FROM:<userx@y.foo.org>
RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org>
will normally be sent directly on to host d.bar.org with envelope
commands
MAIL FROM:<userx@y.foo.org>
RCPT TO:<userc@d.bar.org>
As provided in appendix C, xyz.com MAY also choose to relay the message
to hosta.int, using the envelope commands
MAIL FROM:<userx@y.foo.org>
RCPT TO:<@hosta.int,@jkl.org:userc@d.bar.org>
or to jkl.org, using the envelope commands
MAIL FROM:<userx@y.foo.org>
RCPT TO:<@jkl.org:userc@d.bar.org>
Of course, 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").
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.
Syntax:
"RCPT TO:" ("<Postmaster@" domain ">" / "<Postmaster>" / Forward-Path)
[SP Rcpt-parameters] CRLF
4.1.1.4 DATA (DATA)
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.
The mail data is terminated by a line containing only a period, that
is, the character sequence "<CRLF>.<CRLF>" (see section 4.5.2). This
is the end of mail data indication. Note that 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 data, ends the DATA command
itself. 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 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.
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.
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 it is not accepted and a failure reply is returned. In sending
a positive 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.
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 the top 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 time stamp lines.
Details for formation of these lines, including their syntax, is
specified in section 4.4.
Additional discussion about the operation of the DATA command appears
in section 3.3.
Syntax:
"DATA" CRLF
4.1.1.5 RESET (RSET)
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., if 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. In other situations, it restores the state to
that immediately after the most recent EHLO. 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).
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.
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.
Syntax:
"RSET" CRLF
4.1.1.6 VERIFY (VRFY)
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.
This command has no effect on the reverse-path buffer, the forward-path
buffer, or the mail data buffer.
Syntax:
"VRFY" SP String CRLF
4.1.1.7 EXPAND (EXPN)
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.
This command has no effect on the reverse-path buffer, the forward-path
buffer, or the mail data buffer and may be issued at any time.
Syntax:
"EXPN" SP String CRLF
4.1.1.8 HELP (HELP)
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.
This command has no effect on the reverse-path buffer, the forward-path
buffer, or the mail data buffer and may be issued at any time.
SMTP servers SHOULD support HELP without arguments and MAY support it
with arguments.
Syntax:
"HELP" [ SP String ] CRLF
4.1.1.9 NOOP (NOOP)
This command does not affect any parameters or previously entered
commands. It specifies no action other than that the receiver send an
OK reply.
This command has no effect on the reverse-path buffer, the forward-path
buffer, or the mail data buffer and may be issued at any time. If a
parameter string is specified, servers SHOULD ignore it.
Syntax:
"NOOP" [ SP String ] CRLF
4.1.1.10 QUIT (QUIT)
This command specifies that the receiver MUST send an OK reply, and
then close the transmission channel.
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 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 cancel any 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).
The QUIT command may be issued at any time.
Syntax:
"QUIT" CRLF
4.1.2 Command Argument Syntax
The syntax of the argument fields of the above commands (using the
syntax specified in [ABNF] 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 (section 6) of [ABNF] or in the syntax of [MSGFMT].
Reverse-path = Path
Forward-path = Path
Path = "<" [ A-d-l ":" ] Mailbox ">"
A-d-l = At-domain *( "," A-d-l )
; Note that this form, the so-called "source route",
; MUST BE accepted, SHOULD NOT be generated, and SHOULD be
; ignored.
At-domain = "@" domain
Mail-parameters = esmtp-param *(SP esmtp-param)
Rcpt-parameters = esmtp-param *(SP esmtp-param)
esmtp-param = esmtp-keyword ["=" esmtp-value]
esmtp-keyword = (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
esmtp-value = 1*(%d33-60 / %d62-127)
; any CHAR excluding "=", SP, and control
; characters
Keyword = Ldh-str
Argument = Atom
Domain = (sub-domain 1*("." sub-domain)) / address-literal
sub-domain = Let-dig [Ldh-str]
address-literal = "[" IPv4-address-literal /
IPv6-address-literal /
General-address-literal "]"
; See section 4.1.3
Mailbox = Local-part "@" Domain
Local-part = Dot-string / Quoted-string
; MAY be case-sensitive
Dot-string = Atom *("." Atom)
Atom = 1*atext
Quoted-string = DQUOTE *qcontent DQUOTE
String = Atom / Quoted-string
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.
Systems MUST NOT define mailboxes in such a way as to require the use
in SMTP of non-ASCII characters (octets with the high order bit set 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.
Note that the backslash, "\", is a quote character, which is used to
indicate that the next character is to be used literally (instead of
its normal interpretation). For example, "Joe\,Smith" indicates a
single nine character user field with the comma being the fourth
character of the field.
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-1015]), characters outside
the set of alphas, 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.
4.1.3 Address Literals
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
space, and the address itself, in a format specified as part of the
IPv6 standards [IPv6AddrSpec].
Specifically:
IPv4-address-literal = Snum 3("." Snum)
IPv6-address-literal = "IPv6:" IPv6-addr
General-address-literal = Standardized-tag ":" 1*dcontent
Standardized-tag = Ldh-str
; MUST be specified in a standards-track RFC
; and registered with IANA
Snum = 1*3DIGIT ; representing a decimal integer
; value in the range 0 through 255
Let-dig = ALPHA / DIGIT
Ldh-str = *( ALPHA / DIGIT / "-" ) Let-dig
IPv6-addr = IPv6-full / IPv6-comp / IPv6v4-full / IPv6v4-comp
IPv6-hex = 1*4HEXDIG
IPv6-full = IPv6-hex 7(":" IPv6-hex)
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
IPv6v4-full = IPv6-hex 5(":" IPv6-hex) ":" IPv4-address-literal
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
4.1.4 Order of Commands
There are restrictions on the order in which these commands may be used.
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.
An EHLO command MAY be issued by a client later in the session. If it
is issued after the session begins, 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.
If the EHLO command is not acceptable to the SMTP server, 501, 500, or
502 failure replies MUST be returned as appropriate. The SMTP server
MUST stay in the same state after transmitting these replies that it
was in before the EHLO was received.
The SMTP client MUST, if possible, ensure that the domain parameter to
the EHLO command is a valid principal host name (not a CNAME or MX
name) for its host. 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 and
supplemental information provided that will assist in identifying the
client.
An SMTP server MAY verify that the domain name parameter in the EHLO
command actually corresponds to the IP address of the client. However,
the server MUST NOT refuse to accept a message for this reason if the
verification fails: the information about verification failure is for
logging and tracing only.
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.
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.
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
(or a new EHLO) 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 sent only if no mail
transaction had been started in the session, or it the previous one
successfully concluded with a successful DATA command, or if the
previous one was aborted with a RSET.
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 reply
MUST be returned and the SMTP server MUST stay in the same state.
The last command in a session MUST be the QUIT command. The QUIT
command cannot be used at any other time in a session, but SHOULD be
used by the client SMTP to request connection closure, even when no
session opening command was sent and accepted.
4.1.5 Private-use Commands
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.
Commands sent or accepted by SMTP systems that do not start with "X"
MUST conform to the requirements of section 2.2.2.
4.2 SMTP Replies
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.
The details of the command-reply sequence are described in section 4.3.
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 for 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 section 4.2.1). Since, in violation of this
specification, the text is sometimes not sent, clients which 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 circumstances, however, multiline
replies are allowed for any command.
In ABNF, server responses are:
Greeting = "220 " Domain [ SP text ] CRLF
Reply-line = Reply-code [ SP text ] CRLF
where "Greeting" appears only in the 220 response that announces that
the server is opening its part of the connection.
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.
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.
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 first digit only.
4.2.1 Reply Code Severities and Theory
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 may be present is
reserved for the finest gradation of information.
There are five values for the first digit of the reply code:
1yz Positive Preliminary reply
The command has been accepted, but the requested action is being
held in abeyance, pending confirmation of the information in this
reply. The SMTP client should send another command specifying
whether to continue or abort the action. Note: unextended SMTP does
not have any commands that allow this type of reply, and so does not
have continue or abort commands.
2yz Positive Completion reply
The requested action has been successfully completed. A new request
may be initiated.
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).
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 is
encouraged to try again. A rule of thumb to determine whether a
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.)
5yz Permanent Negative Completion reply
The command was not accepted and the requested action did not occur.
The SMTP client is discouraged from repeating 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
reinitiate the command sequence by direct action at some point in
the future (e.g., after the spelling has been changed, or the user
has altered the account status).
The second digit encodes responses in specific categories:
x0z Syntax: These replies refer to syntax errors, syntactically
correct commands that do not fit any functional category, and
unimplemented or superfluous commands.
x1z Information: These are replies to requests for information, such
as status or help.
x2z Connections: These are replies referring to the transmission
channel.
x3z Unspecified.
x4z Unspecified.
x5z Mail system: These replies indicate the status of the receiver
mail system vis-a-vis the requested transfer or other mail system
action.
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.
For example, a command such as NOOP, whose successful execution does
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.
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.
The format for multiline replies requires that every line, except the
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.
For example:
123-First line
123-Second line
123-234 text beginning with numbers
123 The last line
In many cases the SMTP client then simply needs to search for the reply
code followed by <SP> at the beginning of a line, and ignore all
preceding lines. In a few cases, there is important data for the
client in the reply "text". The client will be able to identify these
cases from the current context.
4.2.2 Reply Codes by Function Groups
500 Syntax error, command unrecognized
(This may include errors such as command line too long)
501 Syntax error in parameters or arguments
502 Command not implemented (see section 4.2.4)
503 Bad sequence of commands
504 Command parameter not implemented
211 System status, or system help reply
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)
220 <domain> Service ready
221 <domain> Service closing transmission channel
421 <domain> Service not available, closing transmission channel
(This may be a reply to any command if the service knows it
must shut down)
250 Requested mail action okay, completed
251 User not local; will forward to <forward-path>
(See section 3.4)
252 Cannot VRFY user, but will accept message and attempt
delivery
(See section 3.5.3)
450 Requested mail action not taken: mailbox unavailable
(e.g., mailbox busy)
550 Requested action not taken: mailbox unavailable
(e.g., mailbox not found, no access, or command rejected
for policy reasons)
451 Requested action aborted: error in processing
551 User not local; please try <forward-path>
(See section 3.4)
452 Requested action not taken: insufficient system storage
552 Requested mail action aborted: exceeded storage allocation
553 Requested action not taken: mailbox name not allowed
(e.g., mailbox syntax incorrect)
354 Start mail input; end with <CRLF>.<CRLF>
554 Transaction failed (Or, in the case of a connection-opening
response, "No SMTP service here")
4.2.3 Reply Codes in Numeric Order
211 System status, or system help reply
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)
220 <domain> Service ready
221 <domain> Service closing transmission channel
250 Requested mail action okay, completed
251 User not local; will forward to <forward-path>
(See section 3.4)
252 Cannot VRFY user, but will accept message and attempt
delivery
(See section 3.5.3)
354 Start mail input; end with <CRLF>.<CRLF>
421 <domain> Service not available, closing transmission channel
(This may be a reply to any command if the service knows it
must shut down)
450 Requested mail action not taken: mailbox unavailable
(e.g., mailbox busy)
451 Requested action aborted: local error in processing
452 Requested action not taken: insufficient system storage
500 Syntax error, command unrecognized
(This may include errors such as command line too long)
501 Syntax error in parameters or arguments
502 Command not implemented (see section 4.2.4)
503 Bad sequence of commands
504 Command parameter not implemented
550 Requested action not taken: mailbox unavailable
(e.g., mailbox not found, no access, or command rejected
for policy reasons)
551 User not local; please try <forward-path>
(See section 3.4)
552 Requested mail action aborted: exceeded storage allocation
553 Requested action not taken: mailbox name not allowed
(e.g., mailbox syntax incorrect)
554 Transaction failed (Or, in the case of a connection-opening
response, "No SMTP service here")
4.2.4 Reply Code 502
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.
4.2.5 Reply Codes After DATA and the Subsequent <CRLF>.<CRLF>
When an SMTP server returns a positive completion status (2yz code)
after the DATA command is completed with <CRLF>.<CRLF>, it accepts
responsibility for:
- delivering the message (if the recipient mailbox exists), or
- 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.
- 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).
When an SMTP server returns a transient error completion status (4yz) code
after the DATA command is completed with <CRLF>.<CRLF>, it MUST NOT make
any subsequent attempt to deliver that message. The SMTP client retains
responsibility for 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).
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.
I.e., if the client SMTP successfully handles these conditions, the user
will not receive such a reply.
When an SMTP server returns a permanent error status (5yz) code after the
DATA command is completely 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 and
intervention of the message.
4.3 Sequencing of Commands and Replies
4.3.1 Sequencing Overview
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.
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 4.1.3 for a discussion of alternatives in these
situations.
For example,
220 ISIF.USC.EDU Service ready
or
220 mail.foo.com SuperSMTP v 6.1.2 Service ready
or
220 [10.0.0.1] Clueless host service ready
The table below lists alternative success and failure replies for each
command. These SHOULD be strictly adhered to: a receiver may
substitute text in the replies, but the meaning and action implied by
the code numbers and by the specific command reply sequence cannot be
altered.
4.3.2 Command-Reply Sequences
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 2 and 5 inclusive.
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.
In addition to the codes listed below, any SMTP command can return any
of the following codes if the corresponding unusual circumstances are
encountered:
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.
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.
421 Service shutting down and closing transmission channel
Specific sequences are:
CONNECTION ESTABLISHMENT
S: 220
E: 554
EHLO or HELO
S: 250
E: 504, 550
MAIL
S: 250
E: 552, 451, 452, 550, 553, 503
RCPT
S: 250, 251 (but see section 3.4 for discussion of 251 and 551)
E: 550, 551, 552, 553, 450, 451, 452, 503, 550
DATA
I: 354 -> data -> S: 250
E: 552, 554, 451, 452
E: 451, 554, 503
RSET
S: 250
VRFY
S: 250, 251, 252
E: 550, 551, 553, 502, 504
EXPN
S: 250, 252
E: 550, 500, 502, 504
HELP
S: 211, 214
E: 502, 504
NOOP
S: 250
QUIT
S: 221
4.4 Trace Information
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 discussed in
section 4.1.1.4.
This line MUST be structured as follows:
- The FROM field, 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.
- The ID field MAY contain an "@" as suggested in RFC-822, but this is
not required.
- The FOR field MAY contain a list of <path> entries when multiple
RCPT commands have been given. This may raise some security issues
and is usually not desirable; see section 7.2.
An Internet mail program MUST NOT change a Received: line that was
previously added to the message header. 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.
As the Internet grows, comparability of Received fields is important
for detecting problems, especially slow relays. SMTP servers that
create Received fields SHOULD use explicit offsets in the dates (e.g.,
-0800), rather than time zone names of any type. Local time (with an
offset) is preferred to UT when feasible. This formulation allows
slightly more information about local circumstances to be specified.
If UT is needed, the 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 a time zone name is used, it SHOULD be
included in a comment.
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 enviroment. Normally, this would mean it had been delivered to
the destination user or an associated mail drop, but in some cases it
may be further processed and transmitted by another mail system.
It is possible for the mailbox in the return path to be different from
the actual sender's mailbox, for example, if error responses are 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.
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 mail data headers and body [MSGFMT].
It is sometimes difficult for an SMTP server to determine whether or
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.
A message-originating SMTP system SHOULD NOT send a message that
already contains a Return-path header. SMTP servers performing a relay
function MUST NOT inspect the message data, and especially not to the
extent needed to determine if Return-path headers are present. SMTP
servers making final delivery MAY remove Return-path headers before
adding their own.
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.
Historical note: Text in RFC 822 that appears to contradict the use of
the Return-path header (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.
In particular:
- a gateway from SMTP->elsewhere SHOULD insert a return-path header,
unless it is known that the "elsewhere" transport also uses Internet
domain addresses and maintains the envelope sender address
separately.
- a gateway from elsewhere->SMTP SHOULD delete any return-path header
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.
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 MUST be an OK reply. However, the SMTP
server MUST compose and send an "undeliverable mail" notification
message to the originator of the message.
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 is preferred when
possible. All undeliverable mail notification messages are sent using
the MAIL command (even if they result from processing the obsolete
SEND, SOML, or SAML commands) and use a null return path as discussed
in section 3.7.
The time stamp line and the return path line are formally defined as
follows:
Return-path-line = "Return-Path:" FWS Reverse-path <CRLF>
Time-stamp-line = "Received:" FWS Stamp <CRLF>
Stamp = From-domain By-domain Opt-info ";" FWS date-time
; where "date-time" is as defined in [MSGFMT]
but the "obs-" forms, especially two-digit
years, are prohibited in SMTP and MUST NOT be
used.
From-domain = "FROM" FWS Extended-Domain CFWS
By-domain = "BY" FWS Extended-Domain CFWS
Extended-Domain = Domain /
( Domain FWS "(" TCP-info ")" ) /
( Address-literal FWS "(" TCP-info ")"
TCP-info = Address-literal / ( Domain FWS Address-literal )
; Information derived by server from TCP connection
not client EHLO.
Opt-info = [Via] [With] [ID] [For]
Via = "VIA" FWS Link CFWS
With = "WITH" FWS Protocol CFWS
ID = "ID" FWS String / msg-id CFWS
For = "FOR" FWS 1*( Path / Mailbox ) CFWS
Link = "TCP" / Addtl-Link
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.
Protocol = "ESMTP" / "SMTP" / Attdl-Protocol
Attdl-Protocol = Atom ; Additional standard names for protocols
are registered with the Internet Assigned
Numbers Authority (IANA). SMTP servers
SHOULD NOT use unregistered names.
4.5 Additional Implementation Issues
4.5.1 Minimum Implementation
In order to make SMTP workable, the following minimum implementation is
required for all receivers. The following commands MUST be supported to
conform to this specification:
EHLO
HELO
MAIL
RCPT
DATA
RSET
NOOP
QUIT
VRFY
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 which specify a mailbox for postmaster at any of
the domains for which the SMTP s erver provides mail service, as well as
the special case of "RCPT TO:<Postmaster>" (with no domain specification),
MUST be supported.
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 attack or 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 which are not part of such attacks.
4.5.2 Transparency
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:
- 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.
- 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.
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 (octets) data stream, the 7-bit ASCII codes
are transmitted right justified in the octets, with the high order bits
cleared to zero. See 3.7 for special treatment of these conditions in SMTP
systems serving a relay function.
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.
4.5.3 Sizes and Timeouts
4.5.3.1 Size limits and minimums
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-X400] 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 which impose no limits on the length of these
objects should be used.
local-part
The maximum total length of a user name or other local-part is 64
characters.
domain
The maximum total length of a domain name or number is 255
characters.
path
The maximum total length of a reverse-path or forward-path is 256
characters (including the punctuation and element separators).
command line
The maximum total length of a command line including the command
word and the <CRLF> is 512 characters. SMTP extensions may be used
to increase this limit.
reply line
The maximum total length of a reply line including the reply code
and the <CRLF> is 512 characters. More information may be conveyed
through multiple-line replies.
text line
The maximum total length of a text line including the <CRLF> is 1000
characters (not counting the leading dot duplicated for
transparency). This number may be increased by the use of SMTP
Service Extensions.
message content
The maximum total length of a message content (including any message
headers as well as the message body) MUST BE at least 64K octets.
Since the introduction of multimedia mail [RFC-MIME], 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 ([RFC-SIZE]), and SMTP client systems that will
send large messages SHOULD utilize it when possible.
recipients buffer
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 servers MUST NOT, and
delivery SMTP servers SHOULD NOT, perform validation tests on
message headers suggests that rejecting a message based on the total
number of recipients shown in header fields is to be discouraged. A
server which imposes a limit on the number of recipients MUST behave
in an orderly fashion, such as to reject 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.
Errors due to exceeding these limits may be reported by using the reply
codes. Some examples of reply codes are:
500 Line too long.
or
501 Path too long
or
452 Too many recipients (see below)
or
552 Too much mail data.
[RFC-821] 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.
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 addresses
will be removed from the SMTP client's queue. When the client attempts
retransmission of those addresses which received 452 responses, at
least 100 of these will be able to fit in the SMTP server's recipients
buffer. Each retransmission attempt which is able to deliver anything
will be able to dispose of at least 100 of these recipients.
If an SMTP server has an implementation limit on the number of RCPT
commands and this limit is exhausted, it MUST use a response code of 452
(but the client SHOULD also be prepared for a 552, as noted above). If the
server has a configured site-policy limitation on the number of RCPT
commands, it MAY instead use a 5XX response code. This would be most
appropriate if the policy limitation was intended to apply if the total
recipient count for a particular message body were enforced even if that
message body was sent in multiple mail transactions.
4.5.3.2 Timeouts
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 is set
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.
Based on extensive experience with busy mail-relay hosts, the minimum
per-command timeout values SHOULD be as follows:
Initial 220 Message: 5 minutes
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.
MAIL Command: 5 minutes
RCPT Command: 5 minutes
A longer timeout is required if processing of mailing lists and
aliases is not deferred until after the message was accepted.
DATA Initiation: 2 minutes
This is while awaiting the "354 Start Input" reply to a DATA command.
Data Block: 3 minutes
This is while awaiting the completion of each TCP SEND call
transmitting a chunk of data.
DATA Termination: 10 minutes.
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 responsibility
for delivery. See section 6.1 for additional discussion.
An SMTP server SHOULD have a timeout of at least 5 minutes while it is
awaiting the next command from the sender.
4.5.4 Retry Strategies
The common structure of a host SMTP implementation includes user
mailboxes, one or more areas for queuing messages in transit, and one
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.
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.
4.5.4.1 Sending Strategy
The general model for an SMTP client is one or more processes that
periodically attempt to transmit outgoing mail. In a typical system,
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.
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.
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. The
parameters to the retry algorithm MUST be configurable.
A client SHOULD keep a list of hosts it cannot reach and corresponding
connection timeouts, rather than just retrying queued mail items.
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.
The SMTP client can shorten the queuing delay in cooperation with the
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 that
discussed in [RFC-ETRN].
The strategy may be further modified as a result of multiple addresses
per host (see below) to optimize delivery time vs. resource usage.
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 that an 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 retries are repeated for dozens,
or even hundreds, of queued messages to the same host.
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.
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. Implementation of this efficiency feature is strongly
encouraged.
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.
4.5.4.2 Receiving Strategy
The SMTP server SHOULD attempt to keep a pending listen on the SMTP
port 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.
As discussed above, when the SMTP server receives mail from a
particular host address, it could notify the SMTP client to retry any
mail pending for that host address.
4.5.5 Messages with a null reverse-path
There are several types of notification messages which 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, see [RFC 1894]) and also
Message Disposition Notifications (MDNs, see [RFC 2298]). 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.)
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
with a valid, non-null reverse-path.
Implementors of automated email processors should be careful to make
sure that the various kinds of messages with null reverse-path are
handled correctly, in particular such systems SHOULD NOT reply to
messages with null reverse-path.
5. Address Resolution and Mail Handling
Once an SMTP client lexically identifies a domain to which mail will be
delivered for processing (as described in sections 3.6 and 3.7), a DNS
lookup MUST be performed to resolve the domain name (see [RFC-DNS]).
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 and, due to a history of problems, are
generally discouraged. The lookup first attempts to locate an MX
record associated with the name. If a CNAME record is found instead,
the resulting name is processed as if it were the initial name. If no
MX records are found, but an A RR is found, the A RR is treated as if
it was associated with an implicit MX RR, with a preference of 0,
pointing to that host. If one or more MX RRs are found for a given
name, SMTP systems MUST NOT utilize any A RRs associated with that name
unless they are located using the MX RRs; the "implicit MX" rule above
applies only 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.
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. In any case, the SMTP client SHOULD try at least two
addresses.
Two types of information is used to rank the host addresses: multiple
MX records, and multihomed hosts.
Multiple MX records contain a preference indication that MUST be used
in sorting (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.
The destination host (perhaps taken from the preferred MX record) may
be multihomed, in which case the domain name resolver will return a
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 SMTP MUST try them in the order presented.
Although the capability to try multiple alternative addresses is
required, specific installations may want to limit or disable the use
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.
If an SMTP server receives a message with a destination for which it 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.
If it determines that it should relay the message without rewriting the
address, it MUST sort the MX records to determine candidates for
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 returned as undeliverable.
If records do remain, they SHOULD be tried, best preference first, as
described above.
6. Problem Detection and Handling
6.1 Reliable Delivery and Replies by Email
When the receiver-SMTP accepts a piece of mail (by sending a "250 OK"
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.
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, 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 down to its final hop.
For example, suppose that an error notification must be sent for a
message that arrived with:
MAIL FROM:<@a,@b:user@d>
The notification message MUST be sent using:
RCPT TO:<user@d>
Some delivery failures after the message is accepted by SMTP will be
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.
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 [RFC-1047] for
a discussion of this problem.
6.2 Loop Detection
Simple counting of the number of "Received:" headers 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.
6.3 Compensating for Irregularities
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 well-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 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.
The problems associated with ill-formed messages were exacerbated by
the introduction of the split-UA mail reading protocols [RFC-POP2,
RFC-POP3, RFC-IMAP2, RFC-PCMAIL]. These protocols have encouraged the
use of SMTP as a posting 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]). 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.
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.
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 protocol:
- Addition of a message-id field when none appears
- Addition of a date, time or time zone when none appears
- Correction of addresses to proper FQDN format
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.
In all cases, properly-operating clients supplying correct information
are preferred to corrections by the SMTP server. In all cases,
documentation of actions performed by the servers (in trace fields
and/or header comments) is strongly encouraged.
7. Security Considerations
7.1 Mail Security and Spoofing
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 which use
digital signatures (see [RFC-1847] and, e.g., [RFC-PGP] or [RFC-SMIME]).
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, unless they are accompanied by careful handoffs of
responsibility in a carefully-designed trust environment, they remain
inherently weaker than end-to-end mechanisms which use digitally signed
messages rather than depending on the integrity of the transport system.
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 or in which error
(or normal) replies should be directed to a special address. (Systems
that provide convenient ways for users to alter these fields on a
per-message basis should attempt to establish a primary and permanent
mailbox address for the user so that Sender fields within the message
data can be generated sensibly.)
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 an ignorant user who is trying to fake mail.
7.2 "Blind" Copies
Addresses that do not appear in the message headers 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 headers, either as part of trace
headers or as informational or private-extension headers. 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.
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 headers. Receiving
systems SHOULD NOT attempt to deduce such relationships and use them to
alter the headers of the message for delivery. The popular
"Apparently-to" header is a violation of this principle as well as a
common source of unintended information disclosure and SHOULD NOT be
used.
7.3 VRFY, EXPN, and Security
As discussed in section 3.5, individual sites may want to disable one
or both VRFY or EXPN for security reasons. 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.
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.
Within the last few years, the contents of mailing lists have become
popular as an address information source for so-called "spammers." The
use of EXPN to "harvest" addresses has increased as list administrators
have installed protections against inappropriate uses of the lists
themselves. Implementations SHOULD still provide support for EXPN, but
sites SHOULD carefully evaluate the tradeoffs. As authentication
mechanisms are introduced into SMTP, some sites may choose to make EXPN
available only to authenticated requestors.
7.4 Information Disclosure in Announcements
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 useful
information to 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 are strongly encouraged to minimally provide for making
type and version information available in some way to other network
hosts.
7.5 Information Disclosure in Trace Fields
In some circumstances, such as when mail originates from within a LAN
whose hosts are not directly from the public Internet, trace
("Received") 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.
7.6 Information Disclosure in Message Forwarding
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.
7.7 Scope of Operation of SMTP Servers
It is a well-established principle that an SMTP server may refuse to
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.
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, MAIL, or RCPT as appropriate.
8. IANA Considerations
IANA will maintain three registries in support of this specification.
The first 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.
The second registry consists of "tags" that identify forms of domain
literals other than those for IPv4 addresses (specified in RFC 821 and
in this document) and IPv6 addresses (specified in this document).
Additional literal types require standardization before being used;
none are anticipated at this time.
The third, 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") 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.
9. References
[US-ASCII] American National Standards Institute (formerly United States of
America Standards Institute), X3.4, 1968, "USA Code for Information
Interchange". ANSI X3.4-1968 has been replaced by newer versions with
slight modifications, but the 1968 version remains definitive for the
Internet.
[RFC-1123] Braden, R., "Requirements for Internet hosts - application and
support", 10/01/1989
[RFC-POP2] Butler, M., D. Chase, J. Goldberger, J. Postel, J. Reynolds,
"Post Office Protocol - version 2", RFC 937, 02/01/1985
[RFC-PGP] Callas, J., L. Donnerhacke, H. Finney, R. Thayer, "OpenPGP
Message Format", RFC 2440, November 1998.
[RFC-IMAP2] Crispin, M., "Interactive Mail Access Protocol - Version 2", RFC
1176, 08/20/1990.
[RFC-IMAP4] Crispin, M., "Internet Message Access Protocol - Version 4", RFC
2060, 12/04/1996.
[RFC-822] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", RFC 822, Department of Electrical Engineering, University of
Delaware, August 1982.
[ABNF] Crocker, D., P. Overell, Eds., "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[RFC-ETRN] De Winter, J., "SMTP Service Extension for Remote Message Queue
Starting", RFC 1985, 08/14/1996.
[IAB-Firewalls] Freed, N, ed., "Behavior of and Requirements for
Internet Firewalls", Work in progress, draft-iab-firewall-req-02.txt,
June 2000.
[RFC-MIME] Freed, N., N. Borenstein, "Multipurpose Internet Mail Extensions
(MIME) Part One: Format of Internet Message Bodies", RFC 2045, 12/02/1996.
[RFC-PIPELINE] N. Freed, A. Cargille, "SMTP Service Extension for Command
Pipelining", RFC 1854, 10/04/1995.
[RFC-1847] Galvin, J., S. Murphy, S. Crocker, N. Freed. "Security
Multiparts for MIME: Multipart/Signed and Multipart/Encrypted", RFC 1847,
October 1995.
[SUBMIT] R. Gellens, J. Klensin, "Message Submission", RFC 2476, December
1998.
[RFC-X400] S. Hardcastle-Kille, "Mapping between X.400(1988) / ISO 10021
and RFC 822", RFC 1327, 05/18/1992.
[IPv6AddrSpec] Hinden, R and S. Deering, Eds. "IP Version 6 Addressing
Architecture", RFC 1884, December 1995.
[RFC-SIZE] J. Klensin, N. Freed, K. Moore, "SMTP Service Extension for
Message Size Declaration", RFC 1870, 11/06/1995. (STD 10)
[SMTPEXT] J. Klensin, N. Freed, M. Rose, E. Stefferud, D. Crocker, "SMTP
Service Extensions", RFC-1869, 11/06/1995. (STD 10)
[8BITMIME] J. Klensin, N. Freed, M. Rose, E. Stefferud, D. Crocker, "SMTP
Service Extension for 8bit-MIMEtransport", RFC 1652, 07/18/1994.
[RFC-PCMAIL] M. Lambert, "PCMAIL: A distributed mail system for personal
computers", RFC 1056, 06/01/1988.
[RFC-DNS] Mockapetris, P., "Domain names - implementation and
specification", RFC 1035 and P. Mockapetris, "Domain names - concepts and
facilities", RFC 1034. (STD 13)
[RFC-INTLHDR] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
Three: Message Header Extensions for Non-ASCII Text", RFC 2047, 12/02/1996.
[RFC-NOTARY1] Moore, K., "SMTP Service Extension for Delivery Status
Notifications", RFC 1891, 01/15/1996.
[RFC-NOTARY2] Moore, K., G. Vaudreuil, "An Extensible Message Format for
Delivery Status Notifications", RFC 1894, 01/15/1996.
[RFC-POP3] Myers, J., M. Rose, "Post Office Protocol - Version 3", RFC 1930,
5/14/96 (Std 53).
[RFC-974] Partridge, C., "Mail routing and the domain system", RFC 974,
01/01/1986
[RFC-1047] Partridge, C., "Duplicate messages and SMTP", RFC 1047,
02/01/1988.
[TCP] Postel, J., ed., "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", RFC 793, USC/Information Sciences
Institute, NTIS AD Number A111091, September 1981.
[RFC-821] Postel, J., "Simple Mail Transfer Protocol", RFC 821, August 1,
1982.
[RFC-SMIME] Ramsdell, B., Ed., "S/MIME Version 3 Message Specification", RFC
2633, June 1999.
[MSGFMT] Resnick, P., Work in progress, draft-ietf-drums-msg-fmt-08.txt,
January 2000.
[RFC-BDAT] Vaudreuil, G., "SMTP Service Extensions for Transmission of Large
and Binary MIME Messages", RFC 1830, 08/16/1995.
[RFC-REPLY] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 1893,
01/15/1996.
10. Editor's Address
John C. Klensin
AT&T Laboratories
Tel: 617-574-3076
email: klensin@research.att.com
11. Acknowledgments
Many people worked long and hard on the many iterations of this document.
There was wide-ranging debate on the mailing list about many technical
issues, and many contributors helped form the wording in this
specification. The hundreds of participants in the many discussions since
RFC 821 was produced are too numerous to mention, but they all helped this
document become what it is.
APPENDICES
A. TCP Transport Service
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, but not in SMTP commands or responses.
B. Generating SMTP Commands from RFC 822 Headers
Some systems use RFC 822 headers (only) in a mail submission protocol,
or otherwise generate SMTP commands from RFC 822 headers 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 a mail envelopes is not
separated early in processing from header information (and kept
separate).
It is recommended that the UA provide its initial MTA with an envelope
separate from the message itself. However, if the envelope is not
supplied, SMTP commands SHOULD be generated as follows:
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 fields SHOULD then be removed
from the headers. Once this process is completed, the remaining
headers SHOULD be checked to verify that at least one To:, Cc:, or
Bcc: header remains. If none do, then a bcc: header with no
additional information SHOULD be inserted as specified in [MSGFMT].
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 field that was already there SHOULD be removed.)
Systems may provide a way for submitters to 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.
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.
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 headers or the foreign system's envelope.
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-only remailer, loops back to the Internet
environment (and the mailing list) are almost inevitable.
C. Source Routes
The <reverse-path> is a reverse source routing list of hosts and a
source mailbox. The first host in the <reverse-path> SHOULD be the
host sending the MAIL command. 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; while servers
MUST be prepared to receive and handle them as discussed in section 3.3
and F.2, clients SHOULD NOT transmit them.
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 is an absolute
address, and the route is information about how to get there. The two
concepts should not be confused.
If source routes are used, RFC 821 and the text below should be
consulted for the mechanisms for constructing and updating the forward-
and reverse-paths.
The SMTP server transforms the command arguments by moving its own
identifier (its domain name or that of any domain for which it is
acting as a mail exchanger), if it appears, from the forward-path to
the beginning of the reverse-path.
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.
Conversely, SMTP servers MUST NOT derive final message delivery
information from message header fields.
When the list of hosts is present, it is a "reverse" source route and
indicates that the mail was relayed through each host on the list (the
first host in the list was the most recent relay). This list is used
as a source route to return non-delivery notices to the sender. As each
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).
D. Scenarios
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.
D.1 A Typical SMTP Transaction Scenario
This SMTP example shows mail sent by Smith at host bar.com, 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.
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
D.2 Aborted SMTP Transaction Scenario
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
D.3 Relayed Mail Scenario
Step 1 -- Source Host to Relay Host
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:<@foo.com: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
Step 2 -- Relay Host to Destination Host
S: 220 xyz.com Simple Mail Transfer Service Ready
C: EHLO foo.com
S: 250 xyz.com is on the air
C: MAIL FROM:<@foo.com: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
D.4 Verifying and Sending Scenario
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: VRFY Crispin
S: 250 Mark Crispin <Admin.MRC@foo.com>
C: SEND 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
E. Other Gateway Issues
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 envelope
information from one system to the message headers or body of another)
have generally proven to be inadequate in important ways. Systems
translating between environments that do not support both envelopes and
headers and Internet mail must be written with the understanding that
some information loss is almost inevitable.
F. Deprecated Features of RFC 821
A few features of RFC 821 have proven to be problematic and SHOULD NOT
be used in Internet mail.
F.1 TURN
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 authenticate the client.
F.2 Source Routing
RFC 821 utilized the concept of explicit source routing to get mail
from one host to another via a series of relays. The requirement to
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.
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.
Clients SHOULD NOT utilize explicit source routing except under unusual
circumstances, such as debugging or potentially relaying around
firewall or mail system configuration errors.
F.3 HELO
As discussed in sections 3.1 and 4.1.1, EHLO is strongly preferred to
HELO when the server will accept the former. Servers must continue to
accept and process HELO in order to support older clients.
F.4 #-literals
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.
F.5 Dates and Years
When dates are inserted into messages by SMTP clients or servers (e.g.,
in trace fields), four-digit years MUST BE used. Two-digit years are
deprecated; three-digit years were never permitted in the Internet mail
system.
F.6 Sending versus Mailing
In addition to specifying a mechanism for delivering messages to user's
mailboxes, RFC 821 provided additional, optional, commands to 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.
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.
X. Change Summary and Loose Ends (Temporary)
X.1 Change summary
X.1.1 Substantive changes between draft-ietf-drums-smtpupd-00.txt and
draft-ietf-drums-smtpupd-01.txt
(i) Slightly clarified the discussions of rejection and failure of VRFY
requests and the associated response codes.
(ii) Slightly clarified the discussion of deferred address validation.
(iii) Removed the IPCE terminology and modified the text in section
4.1.1.2 to explicitly introduce the "mail gateway" terminology and to
begin to distinguish a mail gateway from a conventional relay.
(iv) Explicitly noted that SMTP clients for things like POP and IMAP
may send everything to a single relay for further processing, rather
than resolving final domain names.
(v) Tightened the RSET discussion.
(vi) Deprecation of 251 only for RCPT (still ok for VRFY)
X.1.2. Substantive changes between draft-ietf-drums-smtpupd-01.txt and
draft-ietf-drums-smtpupd-02.txt.
Incorporated additional RFC 1123 material; reorganized several sections
for clarity. Added definitions and other previous "loose end" material.
X.1.3. Substantive changes between draft-ietf-drums-smtpupd-02.txt and
draft-ietf-drums-smtpupd-03.txt.
(i) Eliminated a number of placeholders and tightened some of the
definitions in section 2. Added a few new placeholders for consistency
checking against other documents.
(ii) Removed the state diagrams, per direction at IETF Montreal.
(iii) Added new section 6.3, an attempt to summarize WG discussions on
the "posting" versus "delivery" versus "relay" functions of SMTP and on
whether "fixups" are appropriate in different cases.
(iv) Inserted section 6.1, a minor rewrite of section 5.3.3 of RFC1123.
(v) Added new text to 3.5.5 to discuss the spammer - EXPN relationship.
(vi) The "ASCII requirement" in 4.1.1.4 has been tightened somewhat.
(v) The remaining miscellaneous changes agreed to in Montreal have been
incorporated except as noted below.
X.1.4. Substantive changes between draft-ietf-drums-smtpupd-03.txt and
draft-ietf-drums-smtpupd-04.txt.
Many small changes have been made between these two versions; the list
that follows is not exhaustive.
(i) To clarify some of the text, definitions have been introduced to
distinguish among originating, delivery, relay, and gateway SMTP
systems.
(ii) The role of LF-terminated lines has been clarified.
(iii) Several changes have been made to clarify the principle that, no
matter what originating and final delivery systems might do, relay
systems are not permitted to tamper with message content, even to "fix"
headers that are determined to be invalid. If they deem message
content to be seriously unacceptable, they are encouraged to reject the
messages in preference to trying to fix them up, but, in general, the
theme is "don't look/ don't tell".
(iv) A few more definitions have been added to the terminology section,
and the separate glossary has been eliminated.
(v) I have taken a shot at text to address some of the controversies
that have raged on the WG mailing list (e.g., sections 7.4 and 7.5).
Since there was no consensus on most of those topics, I expect that the
inserted text will satisfy no one except, perhaps, for agreement that
saying nothing would have been worse. As a mechanism for moving
forward, the text in these controversial areas that now appears will be
considered "base"; alterations will be made only if clear consensus
emerges.
(vi) Per discussion in Los Angeles, source routes have been further
deprecated.
(vii) Some of the VRFY/EXPN materials have been moved to "security
considerations", where they appear to belong, some text has been added,
and the conformance statements adjusted to reflect what I perceive to
be WG consensus.
(viii) New MX resolution material has been added to section 5. While
most of this material is from RFC974, the rules have been further
tightened to reflect current practice and experience (974 is written in
a somewhat speculative fashion for a standard). In particular, the
behavior of trying the target host's A RR when MXs existed but all of
them were eliminated is now prohibited, which seems necessary if
another of other ideas being recommended or considered are to be
feasible.
X.1.5. Substantive changes between draft-ietf-drums-smtpupd-04.txt and
draft-ietf-drums-smtpupd-05.txt.
(i) All normative references to RFC 1123 have been removed from the
main body of the text (some still appear in the appendices where they
will remain).
(ii) Section 3.5 has been renamed slightly to distinguish between
"debugging of SMTP implementations" and "debugging of addresses".
Better terminology would be welcome.
(iii) Error conditions resulting from the DATA command have been
clarified.
(iv) Section 4.2 (SMTP replies) has been revised and tightened to
reflect reality and recent discussion on the list.
(v) Appendix E has been revised a bit and moved into section 4.2.1.
Given the importance of the "check only first digit" rule, it has to be
there.
(vi) Added new text for "no SMTP service supported" to sections 3.1,
4.2.2, 4.2.3, and 4.3.2. As noted in 3.1, I'd rather add 521 (which
would work perfectly with the model) rather than overloading 554.
(vii) The Return-path language in section 4.4 has been cleaned up a bit.
(viii) Tightened the "postmaster" language in 4.5.1, requiring a small
change to 4.1.1.3.
(ix) I have unilaterally (with a little help from my friends),
increased some of the size limits. 64 was much too short for a domain
name, and the DNS limit of 255 (?) has now been inserted. That leaves
the return path much too short, but I haven't fixed it (maybe that will
cause us to get rid of them). We still have a 64 character limit on
the local-part, which is also *much* too short. Votes for 128 or longer
limits accepted. See X.1.6(I)
(x) The text on the "recipients buffer" has been rewritten so that (I
hope) it makes sense and gives some explicit guidance for how clients
and servers should proceed if limits are imposed.
X.1.6. Substantive changes between draft-ietf-drums-smtpupd-05.txt and
draft-ietf-drums-smtpupd-06.txt.
Most of the changes in this revision have been editorial rather than
substantive. Major substantive changes include:
(i) The language about maximum sizes of SMTP command lines has been
reworked, per WG mailing list discussion.
(ii) Several instances of "SHOULD" have been promoted to "MUST" when
the reasons for the weaker rule seemed to have disappeared. In
particular, the requirement that an SMTP implementation support
timeouts has become a MUST. Also, conformance to this specification
requires support of EHLO. Older systems should claim conformance to
the [to-be-historical] 821, not this specification.
X.1.7. Substantive changes between draft-ietf-drums-smtpupd-06.txt and
draft-ietf-drums-smtpupd-07.txt.
(i) Removed "implied RSET" text associated with QUIT, as specified at
the December 1997 IETF.
(ii) Required that servers support EHLO, as specified at the December
1997 IETF.
X.1.8. Substantive changes between draft-ietf-drums-smtpupd-07.txt and
draft-ietf-drums-smtpupd-08.txt.
This version involves mostly editorial work and cleanup of loose ends.
(i) New 7.5 added (old one renumbered) to discuss info disclosure
through Received fields.
(ii) Some character set and minor syntax issues clarified.
(iii) Material on code 571 added (thought this had been done long ago;
slipped through the cracks)
(iv) Many clarifications added as the result of list discussions and
suggestions.
(v) Error code presentation has been restructured.
(vi) ABNF conversion done
(vii) IPv6 address format inserted per RFC 1884, since we could not get
clear agreement on an alternative.
(viii) Trivial, silly, examples removed. Others not yet renumbered.
(ix) 3.5.2 and 4.1.1 altered slightly per Eric Allman's notes. Eric
may not like the way I've done either of these change very much: the
first now makes the distinction between returning an address and
returning other stuff (which was permitted by -06, but the text wasn't
as clear as it should have been): if it looks like an address, it needs
to be an address. Similarly, with 4.1.1, Eric wanted to explicitly
permit/legitimize "DATA <SP> <CRLF>". I see several disadvantages to
doing that, so have inserted language that encourages receivers to
tolerate trailing white space, which may have the same practical effect.
X.1.9. Substantive changes between draft-ietf-drums-smtpupd-08.txt and
draft-ietf-drums-smtpupd-09.txt.
The first ten of these reflect, in order, minuted items from the
Chicago IETF (IETF 42).
(i) Clarification of "MUST", etc., in the context of this document
(section 2.3).
(ii) Altered VRFY text to make implementation a SHOULD (section 3.5.1)
and removed VRFY from the mandatory to implement list (section 4.5.1),
per 42nd IETF (Chicago).
(iii) Clarified that exploders are expected to not purge sender
addresses from lists (section 3.10). Note that the Chicago conclusion
was that this should be a "MUST". I could not figure out how to do
that without absolutely prohibiting removing addresses to prevent
loops, to guard against spammers, or for similar legitimate purposes.
So I have written this as a "SHOULD", with additional "strongly
discouraged" words. If someone still wants a MUST, suggest text.
(iv) Altered text to permit clients that sometimes, or even always,
initiate sessions with HELO, rather than EHLO, to be fully-conforming
(section 3.2). [[ Editor's note: I continue to believe that a client
that does not have any service extension support, even to the extent of
being able to send EHLO and parse the response without doing anything
about it, should not be considered fully-conforming to this spec (as
distinct from 821). Consequently, the new text in 3.2 stops well short
of encouraging clients that don't need service extensions from
preferentially using HELO, and the text in 2.2.1 (which specifies that
the extension mechanisms must be supported) has not been changed.
(v) Per Chicago discussions, the text requiring that QUIT be sent has
not been changed. The text in 4.1.1.10 requiring that the server wait
for QUIT has been changed to a SHOULD. However, the text in 4.1.1.5,
prohibiting close on receipt of RSET and that elsewhere prohibiting
close as a normal response, has not been changed.
(vi) Text has been inserted in 4.1.1 and the text in 4.3.2 altered
slightly to clarify the handling of parameters to RSET, DATA, and QUIT
and to 4.1.1.9 specify semantics for parameters to NOOP. I have
followed the minutes on this although I personally agree with kre's
mailing list comments that the "servers SHOULD reject" decision leads
to silly states. I recommend that the WG review this.
(vii) Per discussion in Chicago, no substantive change has been made to
the specification about underscore characters in domain names (section
4.1.2). However, the text has been altered to more accurately reflect
discussion on the mailing list and the source of the requirement.
(viii) Per discussion in Chicago, no change has been made to the
preference for local time in Received headers.
(ix) Per discussion in Chicago, code 571 has been removed and policy
rejection is now reflected i a 550 code (section 3.7 and the response
code lists).
(x) Per discussion in Chicago, no change has been made to the
specification of use of raw CR or LF.
(xi) In section 4.3, the text has been changed, per comments from Dan
Bernstein and others, to require that clients be able to handle replies
that do not contain text strings. A few other places patched to match.
(xii) In sections 4.1.1.1 and 8, the placeholders have been removed.
(xiii) Per discussion on the mailing list (and specifically James
Berriman's concerns), the text has been clarified (sections 4.1.1.2 and
4.1.4) to prohibit MAIL unless no mail transaction is open. This is a
MUST NOT prohibition -- SHOULD NOT makes no sense if this is the
direction we are going to go. 503 has also been added to the list of
valid responses for "MAIL" in 4.3.1 - it can't be issued before
EHLO/HELO in any event. While it is clear that something should be
said, this may not be the desired outcome (I selected it because it was
conservative and easy given the text that was there already); the WG
should check that the text is as intended.
(xiv) Per discussion on the mailing list, a new section 4.5.5 has been
added to describe null return paths and their handling (forward pointer
from 3.7). The text in 4.5.5 is substantially that suggested by
Norbert Bollow. As with (xiii), there is now clear text, but it may
not be what the WG desires. Please check.
(xv) "all addresses" substituted for "each...in turn" in 3.10.2.
(xvi) Requirement for "<" and ">" around paths clarified in section 3.3
(syntax productions were clear and correct, but not this overview
material).
(xvii) Clarified text in 3.3 to permit post-DATA bounces on policy
matters.
X.1.10. Substantive changes between draft-ietf-drums-smtpupd-09.txt and
draft-ietf-drums-smtpupd-10.txt.
(i) A large series of typos, most of them caught by Philip Hazel,
corrected.
(ii) Residual problems with references to mailboxes, forward, and
reverse paths in 4.1.1.2 and 4.1.1.3 corrected and some text, I hope,
clarified.
(iii) Text added to 4.2.5 to talk about 5yz errors after DATA. This
text should be checked carefully -- it is a proposal and may or may not
reflect WG consensus.
(iv) Upper bound on "seconds" has been changed to 60 (not 61), per list
discussion. Years are still four-digits and will stay that way unless
the list discussion converges on something else. The increase to 60
seconds includes an explicit note about leap seconds.
(v) Text has been inserted to reflect the Orlando consensus about
"QUIT", i.e., the client MUST send a QUIT command and SHOULD wait for
the results before closing the connection. Servers are still not
permitted to close without receiving a QUIT and sending a 221 response
(except, of course, under the usual "unavoidable circumstances", in
which case they should get off a 451 if that is feasible).
(vi) The EHLO response specification has been changed back to reflect
non-advertisement of VRFY and some text implying that VRFY was optional
to support has been removed (WG consensus seemed to be moving in that
direction at one point, and the editor reacted prematurely). This
makes the text compatible with RFC1869 and restores VRFY to its RFC1123
status.
(vii) The text in section 5 has been clarified with regard to what a
relay that receives a message because of its designation as an MX can
do and 3.7 has been slightly modified to point to it.
(viii) New text has been added to 3.7 to clarify the use of "SMTP
server" relays in "dumb" originating clients.
(viii) Small wording changes inserted into 4.1.4 (e.g., insertion of ",
if possible," into the first sentence of the fifth paragraph to
eliminate the apparent conflict with the second sentence).
X.1.11. Substantive changes between draft-ietf-drums-smtpupd-10.txt and
draft-ietf-drums-smtpupd-11.txt. Note that the most significant change
is the insertion of RFC 2234-conforming ABNF throughout. That material
should be checked carefully.
(i) Dumb client text revised again, as discussed on the list and using
the text agreed to there. New text simply notes that the submission
issues are outside the scope of the spec/standard.
(ii) ABNF updated and replaced (thanks, Chris). Some possible issues/
questions:
(ii.1) RFC 1869 permits a NUL octet in the greeting (ehlo-greet in
the syntax). Chris proposes to remove that capability on the
grounds that it has no obvious value and will probably not
work with many servers anyway.
(ii.2) Previous drafts have assumed that the tag to indicate what
is coming in a non-IPv4 address literal will be separated by
the address itself by a space. Chris proposes to change it to
a colon, on the theory that this will cause fewer parsing
problems with existing MTAs and MUAs.
(ii.3) The full IPv6 address syntax has been transposed from
the prose of RFC2373 into ABNF. It can be left out and 2373
cited if the WG prefers.
(iii) Added firewall clarification to section 2.3.8.
(iv) Small clarifications and tightenings in several sections, notably
the end of 3.7, 3.8.3, 4.5.4.2, 7.2, clarifying editorial changes
elsewhere, slightly improved crossreferences, and some redundant
sections stripped out. Many thanks to Graham Klyne for extensive
and specific comments in these areas.
(v) The "TO:" in "RCPT TO:" is part of an argument, not part of the
command. Similarly for "FROM:" in "MAIL FROM:". All of these are
believed to be fixed.
(vi)Section 2.3.5 has been changed to make it clear that restrictions
on the syntax and character set of domain names are part of the mail
system, not an intrinsic limit of the DNS itself. This, and some
other text, are not going to be popular with those working to
internationalize the DNS, but I believe it is important to lay down a
clear baseline and then start making modifications or extensions,
rather than trying to delude ourselves with a DNS equivalent of "just
send 8".
(vii) Clarified the EHLO-> HELO fallback requirement in 3.2. I think
this is consistent with what the WG wanted; if not, I'm sure I'll hear
about it.
X.1.12. Substantive changes between draft-ietf-drums-smtpupd-11.txt and
draft-ietf-drums-smtpupd-12.txt.
(i) DATA failure text in section 3.3 has been corrected and clarified.
(ii) "Daytime" and supporting productions removed and replaced by a
reference to 822bis. The new text prohibits use of the "obs-" forms in
822bis, which have been deprecated in SMTP since RFC1123. Note that
this change creates what I think is the first substantive normative
reference between the two documents in order to gain consistency. We
had earlier tried to not bind the two of them together in this way, but
probably it is worth it.
(iii) Conditions under which mail to Postmaster may be dropped or bounced
have been clarified in section 4.5.1.
(iv) Revised the treatment of 251 and 551. Both are now permitted (again),
but the text has been revised to clarify the restritions and clarifications
on using them. See section 3.4.
(v) A lot of small editorial improvements have been made, including
changing of some section titles for clarity and removal of a few more
redundant paragraphs.
Z. Full Copyright Statement
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