INTERNET-DRAFT John Klensin, Editor
Expires in six months MCI
November 22, 1995
Simple Mail Transfer Protocol
draft-ietf-drums-smtpupd-01.txt
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, and
its working groups. Note that other groups may also distribute working
documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months.
Internet-Drafts may be updated, replaced, or obsoleted by other
documents at any time. It is not appropriate to use Internet-Drafts as
reference material or to cite them other than as a "working draft" or
"work in progress".
To learn the current status of any Internet-Draft, please check the
1id-abstracts.txt listing contained in the Internet-Drafts Shadow
Directories on ds.internic.net (US East Coast), nic.nordu.net (Europe),
ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim).
If consensus is reached on this document, it will be forwarded to the
IESG with the recommendation that it be processed as a Proposed
Standard for mail transport.
[[ Note in Draft: this version of the I-D is very much a work in
progress. It should be read by the WG as a proposal about what
material should go into the draft and how it should be structured. The
editor is painfully aware that there are almost certainly still
inconsistencies in section numbering and the like. More important,
neither is the balance and allocation of material between the "model",
"procedures", and "specifications" sections yet right, nor is the
attempt to consolidate the syntax and semantics sections to make it
easier to find things fully satisfactory.
I have also not yet reworked the examples or state diagrams: For
example, I believe that all of the ".ARPA" domains should be pulled out
and replaced with contemporary examples, that all "HELO" examples
should be replaced by "EHLO" ones, and that all free-text mailed error
replies should be replaced by NOTARY-format messages. The rewrite to
use 822-ish ABNF is not complete and this draft contains an odd mix as
a result -- I'd appreciate help with that from someone who has the time
and patience. This effort has, incidentally, further convinced me that
we should create a completely separate RFC that defines the ABNF,
rather than having everything refer to the not-completely-satifactory
definition in 822. WG input on those issues is critical. The WG
should also decide how much of the explanatory and justification
material (e.g., from 1123) should be included: this draft is very
inconsistent along that dimension. And, of course, the WG should
discuss what is in here that shouldn't be and what should be in here
that the incompetent editor has forgotten.
Sections marked with doubled brackets (e.g., "<<") are explicit
placeholders or known major loose ends.]]
TABLE OF CONTENTS
0. ABSTRACT
1. INTRODUCTION
2. THE SMTP MODEL
2.1 Basic structure
2.2 The extension model
2.3 Other terminology
3. THE SMTP PROCEDURES: AN OVERVIEW
3.1 Session Initiation
3.2 Client initiation
3.3. Mail
3.4. Forwarding for Address Correction or Updating
3.5. Verifying and Expanding
3.6. Sending and Mailing
3.7. Domains
3.8. Relaying
3.9. Changing Roles
3.10. Terminating sessions and connections
4. THE SMTP SPECIFICATIONS
4.1. SMTP Commands
4.1.1. Command Semantics and Syntax
4.1.2. Lower-level Syntax
4.1.3 Order of commands
4.1.4 Private-use commands
4.2. SMTP Replies
4.2.1. Reply Codes by Function Group
4.2.2. Reply Codes in Numeric Order
4.2.3. Reply code 502
4.2.4 Reply codes after DATA and the subsequent CRLF.CRLF.
4.3. Sequencing of Commands and Replies
4.4 Trace information
4.5. State Diagrams
4.6. Details
4.6.1. Minimum Implementation
4.6.2. Transparency
4.6.3. Sizes and Timeouts
4.6.4 Queuing Strategies
5. Problem detection and handling
5.1 Replies by email
5.2 Loop detection
6. Security Considerations
7. References
8. Editor's addresses
9. Acknowledgements
APPENDIX A: TCP
APPENDIX B: Generating SMTP commands from RFC 822 headers
APPENDIX E: Theory of Reply Codes
APPENDIX F: Scenarios
APPENDIX G: Other gateway issues.
APPENDIX H: Glossary
APPENDIX 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, consolodating 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 [RFC974],
* the clarifications and applicability statements in
RFC 1123 [RFC-1123], and
* material drawn from the SMTP Extension mechanisms [SMTPEXT].
It is intended to replace RFC 821, RFC 974, and the mail transport
materials of RFC 1123. However, RFC 821 specifies some features that
are not in significant use in the Internet of the mid-1990s and, in
appendices, some additional transport models. Those sections are
omitted in this document 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 appeared
to need amplification. These have been identified in multiple ways,
mostly by tracking flaming on the header-people list [HEADER-PEOPLE]
and problems of unusual readings or interpretations that have turned
up as the SMTP extensions have been deployed. It is important to
note that everything here is in response to some identified confusion
or bad behavior, not just paranoia.
Where this specification moves beyond consolodation 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 posting" protocol, as recommended for POP [RFC-POP2,
RFC-POP3] and IMAP [RFC-IMAP4].
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 discusses mail bodies and formats: RFC 822,
MIME, and their relationship.
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. A Glossary provides the definitions of terms as used in this
document.
An important feature of SMTP is its capability to transport mail
across transport service environments, usually referred to as "mail
gatewaying". A transport service environment might consist of the
mutually-TCP-accessible hosts on the public internet, a
firewall-isolated private TCP/IP LAN, or a LAN or WAN environment
utilizing an entirely different transport-level protocol. It is
important to realize that transport systems are not one-to-one with
usual definitions of "networks". A process can communicate directly
with another process, and mail communicated, through any mutually
known transport layer. Conversely, mail can be relayed (actually
gatewayed) between hosts on different transport systems by a host on
both transport systems. The Mail eXchanger mechanisms of the domain
name system [RFC-DNS, RFC974] usually permit relaying and gatewaying
to occur invisibly to the user.
2. THE SMTP MODEL
2.1 Basic structure
The SMTP design is based on the following model of communication: as
the result of a user mail request (or transfer from a mail user agent
(see section 2.3), the SMTP client establishes a two-way transmission
channel to an SMTP server. Fully-capable client SMTPs determine the
host address supporting the server SMTP function by resolving the
domain name in the user request to it into either an intermediate
mail exchanger host or a final target host. In other cases, common
with clients associated with implementations of the POP [RFC-POP2,
RFC-POP3] or IMAP [RFC-IMAP4] protocols, or when the client is inside
an isolated transport service enviroment, the SMTP client may send all
of its traffic to a single SMTP server which, in turn, relays the
mail to final (or other intermediate) destinations and which supports
all of the queuing, retrying, and alternate address functions
discussed in this specification. The SMTP server may be either the
ultimate destination or an intermediate (i.e., may assume the role of
an SMTP client after receiving the message). 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.
Once the transmission channel is established and initial handshaking
completed, the SMTP-sender sends a MAIL command indicating the sender
of the mail. If the server SMTP can accept mail it responds with an
OK reply. The client SMTP then sends a RCPT command identifying a
recipient of the mail. If the server SMTP can accept mail for that
recipient (or believes that it can but cannot immediately verify that
fact--see below) it responds with an OK reply; if not, it responds
with a reply rejecting that recipient (but not the whole mail
transaction). The client and server SMTPs may negotiate several
recipients. When the recipients have been negotiated the client
sends the mail data, terminating with a special sequence. If the
server successfully processes the mail data it responds with an OK
reply. Either the sender or recipient commands 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.
-------------------------------------------------------------
+----------+ +----------+
+------+ | | | |
| User |<-->| | SMTP | |
+------+ | Sender- |Commands/Replies| Receiver-|
+------+ | SMTP |<-------------->| SMTP | +------+
| File |<-->| | and Mail | |<-->| File |
|System| | | | | |System|
+------+ +----------+ +----------+ +------+
SMTP client SMTP server
Model for SMTP Use
Figure 1
-------------------------------------------------------------
An SMTP server may "accept mail" for a recipient under one of two
circumstances: when it can actually verify the address or when it can
make a determination that it is willing to accept responsibility for
the mail. Replies to the RCPT command MUST NOT be delayed beyond a
reasonable time in order to verify addresses. Hence, a "250 OK"
reply to a RCPT command does not necessarily imply that the delivery
address(es) are valid. Errors found after message acceptance will be
reported by mailing a notification message to an appropriate address.
DISCUSSION:
The set of conditions under which a RCPT parameter can be
validated immediately is an engineering design choice.
Reporting destination mailbox errors to the Sender-SMTP
before mail is transferred is generally desirable to save
time and network bandwidth, but this advantage is lost if
RCPT verification is lengthy.
For example, most SMTP servers can immediately verify any
simple local reference, such as a single locally-
registered mailbox. On the other hand, the "reasonable
time" limitation generally implies deferring verification
of a mailing list until after the message has been
transferred and accepted, since verifying a large mailing
list can take a very long time. An implementation might
or might not choose to defer validation of addresses that
are non-local and therefore require a DNS lookup. If a
DNS lookup is performed but a soft domain system error
(e.g., timeout) occurs, validity must be assumed. An SMTP
relay would usually defer verification of addresses when
service extensions are specified that require verification
with the destination host.
The SMTP provides mechanisms for the transmission of mail; directly
from the sending user's host to the receiving user's host when the
two hosts are connected to the same transport service, via one or
more relay SMTP-servers when the source and destination hosts are not
connected to the same transport service, or when an intermediate
host is selected via a Mail eXchanger mechanism.
To be able to provide the relay capability the server SMTP is
supplied with the name of the ultimate destination host as well as
the destination mailbox name. Usually, intermediate hosts are
determined via the DNS MX record, not by explicit "source" routing.
The argument to the MAIL command is normally an address in
mailbox@domain format, which specifies who the mail is from. The
argument to the RCPT command is normally also an address, which
specifies who the mail is to. More generally, the MAIL address is
a forward-path and the RCPT address a reverse-path. The
forward-path is a source route, while the reverse-path is a return
route (which may be used to return a message to the sender when an
error occurs with a relayed message).
When the same message is sent to multiple recipients the SMTP
encourages the transmission of only one copy of the data for all the
recipients at the same destination host.
The mail commands and replies have a rigid syntax. Replies also have
a numeric code. In the following, examples appear which use actual
commands and replies. The complete lists of commands and replies
appears in Section 4 on specifications.
Commands and replies are not case sensitive. That is, a command or
reply word MAY be upper case, lower case, or any mixture of upper and
lower case. Note that this is not true of mailbox user names. For
some hosts the user name is case sensitive (this practice impedes
interoperability and is discouraged), and SMTP implementations
MUST take care to preserve the case of user names as they appear in
mailbox arguments. Domain names are not case sensitive.
Commands and replies are composed of characters from the ASCII
character set [1]. 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. SMTP clients 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.
Eight -bit transmission MAY be requested of the server by the
client using extended SMTP facilities.
When specifying the general form of a command or reply, an argument
(or special symbol) will be denoted by a meta-linguistic variable (or
constant), for example, "<string>" or "<reverse-path>". Here the
angle brackets indicate these are meta-linguistic variables.
However, some arguments use the angle brackets literally. For
example, an actual reverse-path is enclosed in angle brackets, i.e.,
"<John.Smith@ISI.EDU>" is an instance of <reverse-path> (the
angle brackets are actually transmitted in the command or reply).
2.2 The Extension Model
2.2.1 Background
In an effort that started in 199??, 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 that goes beyond the original basic
SMTP requirements. SMTP implementations SHOULD support the basic
extension mechanisms (see below for details), i.e., servers
should 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
implementations, SMTP clients and servers MUST support the
original HELO mechanisms.
Although SMTP is widely and robustly deployed, some parts of the
Internet community might wish to extend the SMTP service. The SMTP
extension mechanism defines a means whereby an extended SMTP client
and server may recognize each other as such and the server can inform
the client as to the service extensions that it supports.
It must be emphasized that any extension to the SMTP service should
not be considered lightly. SMTP's strength comes primarily from its
simplicity. Experience with many protocols has shown that:
protocols with few options tend towards ubiquity, whilst
protocols with many options tend towards obscurity.
This means that 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.
Given this environment, the extension framework consists of:
(1) The SMTP command EHLO, superseding the earlier HELO,
(2) a registry of SMTP service extensions, and
(3) additional parameters to the SMTP MAIL FROM and RCPT TO
commands.
2.2.2 Definition and Registration of Extensions
The IANA maintains a registry of SMTP service extensions.
Associated with each such extension is a corresponding EHLO
keyword value. Each service extension registered with the IANA
must be defined in an RFC. Such RFCs must either be on the
standards-track or must define an IESG-approved experimental
protocol. The definition must include:
(1) the textual name of the SMTP service extension;
(2) the EHLO keyword value associated with the extension;
(3) the syntax and possible values of parameters associated
with the EHLO keyword value;
(4) 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);
(5) any new parameters the extension associates with the
MAIL FROM or RCPT TO verbs;
(6) how support for the extension affects the behavior of a
server and client SMTP; and,
(7) the increment by which the extension is increasing the
maximum length of the commands MAIL FROM, RCPT TO, or
both, over that specified in RFC 821.
In addition, any EHLO keyword value that starts with an upper
or lower case "X" refers to a local SMTP service extension,
which is used through bilateral, rather than standardized,
agreement. Keywords beginning with "X" may not be used in a
registered service extension.
Any 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 are bound by the same rules as EHLO keywords;
specifically, verbs begining with "X" are local extensions
that may not be registered or standardized and verbs not
beginning with "X" must always be registered.
2.3 Terminology
A glossary of terms appears at the end of this document. 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.
SMTP relays a mail object containing an envelope and a content.
(1) The SMTP envelope is straightforward, and 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); a delivery mode (e.g., deliver to
recipient mailboxes); and, one or more recipient addresses.
(2) The SMTP content is sent in the SMTP DATA protocol unit
and has two parts: the headers and the body. The
headers form a collection of field/value pairs
structured according to RFC 822 [RFC822], whilst the body,
if structured, is defined according to MIME [3]. The
content is textual in nature, expressed using the US
ASCII repertoire (ANSI X3.4-1986). Although extensions
(such as MIME) may relax this restriction for the
content body, the content headers are always encoded
using the US ASCII repertoire. The algorithm defined in
[4] is used to represent header values outside the US
ASCII repertoire, whilst still encoding them using the
US ASCII repertoire.
<<placeholders>>
SMTP-sender, SMTP-receiver -> client and server
UA
MTA
host
domain
buffer
state table
2.4 Syntax Principles
The commands consist of a command code followed by an argument field.
Command codes are four alphabetic characters. Upper and lower case
alphabetic characters are to be treated identically. Thus, any of
the following may represent the mail command:
MAIL Mail mail MaIl mAIl
This also applies to any symbols representing parameter values, such
as "TO" or "to" for the forward-path. Command codes and the argument
fields are separated by one or more spaces. However, within the
reverse-path and forward-path arguments case is important. In
particular, in some hosts the user "smith" is different from the user
"Smith".
The argument field consists of a variable length character string
ending with the character sequence <CRLF>. The receiver is to take
no action until this sequence is received.
The syntax for each command is shown with the discussion of that
command, with common elements and parameters shown in section
<<>>??.??.
Square brackets denote an optional argument field. If the option is
not taken, the appropriate default is implied.
<<>> Reference 822 ABNF.
3. THE SMTP PROCEDURES: AN OVERVIEW
This section presents the procedures used in SMTP in several parts.
First comes the basic mail procedure defined as a mail transaction.
Following this are descriptions of forwarding mail, verifying mailbox
names and expanding mailing lists, sending to terminals instead of or
in combination with mailboxes, and the opening and closing exchanges.
At the end of this section are comments on relaying, a note on mail
domains, and a discussion of changing roles. Throughout this section
are examples of partial command and reply sequences, several complete
scenarios are presented in Appendix F.
3.1 Session initiation: EHLO
An SMTP session is initiated by the client opening a connection to
the server and the server responding with an opening message.
SMTP server implementations SHOULD include identification of their
software and version information in the connection greeting reply
after the 220 code. This practice permits much more efficient
isolation and repair of any problems. While some systems also
identify their contact point for mail problems, this is not a
substitute for maintaining the required Postmaster address (see
[RFC822]). Implementations MAY make provision for SMTP servers to be
configured to disable the software and version announcement where it
causes security concerns.
3.2 Client initiation: EHLO
The client then sends the EHLO command to the server, indicating its
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, unable to support service extensions, MAY use HELO
instead of EHLO but EHLO SHOULD be used by all current clients and
accepted by all current systems.
In the EHLO, or the older HELO, 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").
-------------------------------------------------------------
Example of Connection Opening
R: 220 BBN-UNIX.ARPA Simple Mail Transfer Service Ready
S: HELO USC-ISIF.ARPA
R: 250 BBN-UNIX.ARPA
Example 5
-------------------------------------------------------------
-------------------------------------------------------------
Example of Connection Closing
S: QUIT
R: 221 BBN-UNIX.ARPA Service closing transmission channel
Example 6
-------------------------------------------------------------
3.3. MAIL
There are three steps to SMTP mail transactions. The transaction
is started 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 gives the
mail data. And finally, the end of mail data indicator confirms
the transaction.
The first step in the procedure is the MAIL command. The
<reverse-path> contains the source mailbox.
MAIL <SP> 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. It gives the
reverse-path 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.
The <reverse-path> can contain more than just a mailbox. The
<reverse-path> is a reverse source routing list of hosts and
source mailbox. The first host in the <reverse-path> should be
the host sending this command.
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 <SP> TO:<forward-path> [<SP> <rcpt-parameters>] <CRLF>
This command gives a forward-path identifying one recipient.
If accepted, the SMTP server returns a 250 OK reply, and
stores the forward-path. If the recipient is unknown the
SMTP server returns a 550 Failure reply (other circumstances
and reply codes are possible). This second step of the procedure
can be repeated any number of times. The <forward-path> can
contain more than just a mailbox. The <forward-path> may be a
source routing list of hosts and the 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. Servers MUST be prepared
to encounter a list of source routes in the forward path, but MAY
ignore the routes or decline to support the relaying they imply.
Similarly, servers MAY decline to accept mail that is destined for
other hosts or systems. Of course, such a restrictions would make
a server useless as a relay for clients that do not support full
SMTP functionality, but such clients MUST NOT assume that any SMTP
server on the Internet can be used as their mail processing site.
Clients SHOULD NOT utilize explicit source routing except under
unusual circumstances, such as debugging or potentially relaying
around firewalls or mail system configuration errors. If
source routes are used, the first host in the <forward-path>
should be the host receiving this command.
The optional <mail-parameters> are associated with negotiated SMTP
service extensions (see section 2.2).
The third step in the procedure is the DATA command.
DATA <CRLF>
If accepted, the SMTP server returns a 354 Intermediate reply
and considers all succeeding lines to be the message text.
When the end of text is received and stored the SMTP-receiver
sends a 250 OK reply.
Since the mail data is sent on the transmission channel the end
of the 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 "." (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 should
fail only if the mail transaction was incomplete (for example,
no recipients), or if resources are not available. However,
some servers in practice 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 <<>>. Using a "recipient
not found" or equivalent reply code after the data are
accepted makes it difficult or impossible for the client to
determine which recipients failed.
The above procedure is an example of a mail transaction. These
commands must be used only in the order discussed above.
Example 1 (below) illustrates the use of these commands in a mail
transaction.
-------------------------------------------------------------
Example of the SMTP Procedure
This SMTP example shows mail sent by Smith at host Alpha.ARPA,
to Jones, Green, and Brown at host Beta.ARPA. Here we assume
that host Alpha contacts host Beta directly.
S: MAIL FROM:<Smith@Alpha.ARPA>
R: 250 OK
S: RCPT TO:<Jones@Beta.ARPA>
R: 250 OK
S: RCPT TO:<Green@Beta.ARPA>
R: 550 No such user here
S: RCPT TO:<Brown@Beta.ARPA>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: <CRLF>.<CRLF>
R: 250 OK
The mail has now been accepted for Jones and Brown. Green did
not have a mailbox at domain Beta.ARPA.
Example 1
-------------------------------------------------------------
3.4. FORWARDING FOR ADDRESS CORRECTION OR UPDATING
The "forwarding" mechanisms described in section 3.2 of RFC 821, and
especially the 251 reply code from RCPT that indicates a corrected
destination, are no longer in active use. Forwarding support is most
often required to consolodate and simplify addresses within, or
relative to, some enterprise. In most of those cases, information
hiding (and sometimes security) considerations argue against exposure
of the "final" address through the SMTP protocol as a consequence of
the forwarding activity and, in some cases, that final address may
not even be reachable by the sender.
Silent forwarding of messages (without server notification to the
sender) is common in the contemporary Internet.
If the forwarding and address correction mechanisms described in
RFC 821 are used, the addresses given should be stable enough that
it would be reasonable for the client to update local records with
them.
3.5. VERIFYING AND EXPANDING
3.5.1 Overview
SMTP provides, as additional features, commands to verify a user
name or expand a mailing list. This is done with the VRFY and
EXPN commands, which have character string arguments. For the
VRFY command, the string is a user name (see below) and the
response may include the full name of the user and must include
the mailbox of the user, e.g., it MUST BE in either
User Name <mailbox@domain>
or
mailbox@domain
form.
Paths (explicit source routes) MUST NOT be returned by VRFY or
EXPN.
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, either of the
following are legitimate response to VRFY:
553 User ambiguous
or
553- Ambiguous; Possibilities are
553-Joe Smith <jsmith@somedomain>
553-Harry Smith <hsmith@somedomain>
553 Melvin Smith <dweep@somedomain>
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, will facilitate automated
translation into other languages as needed.
For the EXPN command, the string identifies a mailing
list, and the multiline response may include the full name of the
users and must give the mailboxes on the mailing list.
"User name" is a fuzzy term and used purposely. An
implementation of the VRFY or EXPN commands MUST include at
least recognition of local mailboxes as "user names". If a
host chooses to recognize other strings as "user names" that is
allowed.
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 can be given if on receipt of a message
so addressed it will be delivered to everyone on the list,
otherwise an error should be reported (e.g., "550 That is a
mailing list, not a user"). If a request is made to expand a user
name a positive response can be formed by returning a list
containing one name, or an error can be reported (e.g., "550 That
is a user name, not a mailing list").
In the case of a 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.
The case of verifying a user name is straightforward as shown in
example 3.
-------------------------------------------------------------
Example of Verifying a User Name
Either
S: VRFY Smith
R: 250 Fred Smith <Smith@USC-ISIF.ARPA>
Or
S: VRFY Smith
R: 251 User not local; will forward to <Smith@USC-ISIQ.ARPA>
Or
S: VRFY Jones
R: 550 String does not match anything.
Or
S: VRFY Jones
R: 551 User not local; please try <Jones@USC-ISIQ.ARPA>
Or
S: VRFY Gourzenkyinplatz
R: 553 User ambiguous.
Example 3
-------------------------------------------------------------
The case of expanding a mailbox list requires a multiline reply as
shown in example 4.
-------------------------------------------------------------
Example of Expanding a Mailing List
Either
S: EXPN Example-People
R: 250-Jon Postel <Postel@USC-ISIF.ARPA>
R: 250-Fred Fonebone <Fonebone@USC-ISIQ.ARPA>
R: 250-Sam Q. Smith <SQSmith@USC-ISIQ.ARPA>
R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
R: 250-<joe@foo-unix.ARPA>
R: 250 <xyz@bar-unix.ARPA>
Or
S: EXPN Executive-Washroom-List
R: 550 Access Denied to You.
Example 4
-------------------------------------------------------------
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 is
a variety of file naming conventions in the Internet.
3.5.2 VRFY normal response.
When normal (2yz or 551) responses are returned from a VRFY or EXPN
request, the reply MUST include the mailbox name, e.g., "<foo@bar>"
(where "bar" is a fully qualified domain name) must appear in the
syntax. 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 should be given.
Server implementations MUST support VRFY and SHOULD support 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, if supported, be
listed in the response to EHLO if service extensions 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 (note
that implementation of VRFY is required by RFC 1123 and EXPN is
strongly recommended; this specification does not change that
requirement and, hence, except as provided in section 3.5.5,
implementations that return 500 or 502 for VRFY are not in compliance
with the specification).
Especially when a server is acting as a mail exchanger for another,
there may be circumstances where an address appears to be correct but
cannot reasonably be verified in real time. In that situation, reply
code 252 SHOULD BE returned. These cases parallel the discussion of
RCPT verification discussed in section 2.1 although implementations
generally SHOULD be more aggressive about address verification in the
case of VRFY than in the case of RCPT even if a little more time is
required to do so.
3.5.4. Semantics and applications of EXPN.
While 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--both for hosts
(typically with MX and CNAME DNS records) and for mailboxes
(various types of local host aliases) has made it nearly
impossible for these strategies to work, and mail systems SHOULD
NOT attempt them.
3.5.5 VRFY, EXPN, and security.
As discussed above, individual sites may want to disable one or both
of 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 SHOULD 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 for syntax only 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.
3.6. SENDING AND MAILING
The main purpose of SMTP is to deliver messages to user's mailboxes.
A very similar service provided by some hosts is to deliver messages
to user's terminals (provided the user is active on the host). The
delivery to the user's mailbox is called "mailing", the delivery to
the user's terminal is called "sending". Because in many hosts the
implementation of sending is nearly identical to the implementation
of mailing these two functions were combined in SMTP as specified in
RFC 821. However the sending commands were not included in the
required minimum implementation (Section 4.5.1) and, indeed, have not
been widely deployed.
Implementations of them, if provided, should refer to the details in
RFC 821. If one or more of the commands (SEND, SAML, SOML) are
implemented, and service extensions are supported, the EHLO command
response MUST list their names.
3.7. DOMAINS
Domains have become a key concept in the Internet
mail system. The use of domains changes the address space from a
flat global space of simple character string host names to a
hierarchically structured rooted tree of global addresses. The
host name is replaced by a domain and host designator which is a
sequence of domain element strings separated by periods with the
understanding that the domain elements are ordered from the most
specific to the most general.
For example, "ISIF.ISI.EDU", "Fred.Cambridge.UK", and
"PC7.LCS.MIT.EDU" might be domain identifiers.
Whenever domain names are used in SMTP, only resolvable,
fully-qualified, domain names (FQDNs) are permitted. In other words,
names that can be resolved to MX RRs or A RRs (as discussed in
section ??.??.??) 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. [[Note in draft: this represents a
liberalization from the provisions of RFC 1123, section 5.2.2 -- WG
please discuss.]] There is one exception to this rule: the domain
name given in the EHLO (or HELO) command MUST BE either a primary
host name (a domain name that resolves to an A RR) or, if the host
has no name, a domain literal in dotted-decimal notation.
3.8. RELAYING
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.
In general, the availability of Mail eXchanger records in the
domain name system [RFC-DNS] 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. They are also permitted to ignore the route
information and simply send to the final destination as specified in
the route and the DNS. However, there has been a practice, albeit
invalid, 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 -- one of several reasons
why SMTP clients MUST NOT generate invalid source routes or depend on
serial resolution of names.
If 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. If source routes are
used, RFC 821 should be consulted for the mechanisms for constructing
and updating the forward- and reverse-paths.
Using source routing the SMTP server receives mail to be relayed to
another SMTP server. The SMTP server may accept or reject the task
of relaying the mail in the same way it accepts or rejects mail for a
local user. 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. The SMTP server
then becomes an SMTP client, establishes a transmission channel to
the next SMTP server in the forward-path, and sends it the mail.
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 message delivery information
from message header fields.
If an SMTP server has accepted the task of relaying the mail and
later finds that the forward-path 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 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 should not send notification
messages about problems with 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 SHOULD BE set to null. A MAIL command
with a null reverse-path appears as follows:
MAIL FROM:<>
An undeliverable mail notification message is shown in example 7.
This notification is in response to a message originated by JOE at
HOSTW and sent via HOSTX to HOSTY with instructions to relay it on
to HOSTZ. What we see in the example is the transaction between
HOSTY and HOSTX, which is the first step in the return of the
notification message.
-------------------------------------------------------------
Example Undeliverable Mail Notification Message
S: MAIL FROM:<>
R: 250 ok
S: RCPT TO:<@HOSTX.ARPA:JOE@HOSTW.ARPA>
R: 250 ok
S: DATA
R: 354 send the mail data, end with .
S: Date: 23 Oct 81 11:22:33
S: From: SMTP@HOSTY.ARPA
S: To: JOE@HOSTW.ARPA
S: Subject: Mail System Problem
S:
<<>>replace with NOTARY format <<>>
S: .
R: 250 ok
Example 7
-------------------------------------------------------------
3.9. CHANGING ROLES
The TURN command was specified in RFC 821 as a mechanism for
reversing the roles of the client and server programs communicating
over the transmission channel. It has proven in practice to cause a
security problem in environments in which the identity of the client
cannot be accurately verified by the server. TURN SHOULD NOT be used
in such environments, which are the norm with SMTP. For details of
TURN, see RFC 821. Since TURN was optional in the original
specification, implementations that support it and also support
service extensions MUST identify TURN in the EHLO reply.
3.10. TERMINATING SESSIONS AND CONNECTIONS
An SMTP connection is terminated by the client's sending a QUIT
command. The server then responds with a positive reply code, after
which it closes the connection.
An SMTP server MUST NOT intentionally close the connection
except:
o After receiving a QUIT connand and responding with a 221 reply.
o After detecting the need to shutdown the SMTP service and
returning a 451 reply to any command.
In particular, a server that closes connections in response
to commands that are not understood is in violation of this
specification. Instead, 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 451 response
code to the SMTP client before exiting. The SMTP client will
normally read the 451 response code after sending its next
command.
[[Note in draft: Keith and Ned suggest that we should invent a new
error code to be sent by the server when it shuts down the connection
because it has timed out waiting for a client command and that it
should be a 5yz code (since nothing temporary is happening). Such a
shutdown is, of course, permitted by RFC 1123 and by good sense. I
have not done this yet because I (and Mark) fear that introducing a
new code could create an excuse for more of the "send code and
shutdown" behavior patterns that we have been trying to eliminate.
Would a 4yz code be a way out? Comments?]]
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 command codes themselves are alphabetic
characters terminated by <SP> if parameters follow and <CRLF>
otherwise. The syntax of mailboxes must conform to receiver site
conventions. 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.
4.1.1.1 HELLO (HELO) or Extended HELLO (EHLO)
These commands are used to identify the SMTP client to the SMTP
server. The argument field contains the host name of the SMTP
client.
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 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.
These commands and an 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.
If the server SMTP implements and is able to perform the EHLO
command, it will return code 250. This indicates that both the
server and client SMTP are in the initial state, that is, there is no
transaction in progress and all state tables and buffers are cleared.
Normally, this response will be a multiline reply. Each line of the
response contains a keyword and, optionally, one or more parameters.
The syntax for a positive response, using the ABNF notation of
[RFC822], is:
ehlo-ok-rsp ::= "250" domain [ SP greeting ] CR LF
/ ( "250-" domain [ SP greeting ] CR LF
*( "250-" ehlo-line CR LF )
"250" SP ehlo-line CR LF )
; the usual HELO chit-chat
greeting ::= 1*<any character other than CR or LF>
ehlo-line ::= ehlo-keyword *( SP ehlo-param )
ehlo-keyword ::= (ALPHA / DIGIT) *(ALPHA / DIGIT / "-")
; syntax and values depend on ehlo-keyword
ehlo-param ::= 1*<any CHAR excluding SP and all
control characters (US ASCII 0-31
inclusive)>
ALPHA ::= <any one of the 52 alphabetic characters
(A through Z in upper case, and,
a through z in lower case)>
DIGIT ::= <any one of the 10 numeric characters
(0 through 9)>
CR ::= <the carriage-return character
(ASCII decimal code 13)>
LF ::= <the line-feed character
(ASCII decimal code 10)>
SP ::= <the space character
(ASCII decimal code 32)>
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
begun in RFC 821.
4.1.1.2 MAIL (MAIL)
This command is used to initiate a mail transaction in which the mail
data is delivered to one or more mailboxes. The argument field
contains a reverse-path.
The reverse-path consists of an optional list of hosts and the sender
mailbox. 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). In
some types of error reporting messages (for example, undeliverable
mail notifications) the reverse-path may be null (see Example 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> [ <mail-parameters> ]
or
MAIL FROM:<>
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 forward-path consists of an optional list of hosts and a
required destination mailbox. When the list of hosts is
present, it is a source route and indicates that the mail
must be relayed to the next host on the list. If the
SMTP server does not implement the relay function it may
user the same reply it would for an unknown local user
(550).
When mail is relayed, the relay host must remove itself from
the beginning forward-path and put itself at the beginning
of 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 A with arguments
FROM:<USERX@HOSTY.ARPA>
TO:<@HOSTA.ARPA,@HOSTB.ARPA:USERC@HOSTD.ARPA>
will be relayed on to host B with arguments
FROM:<@HOSTA.ARPA:USERX@HOSTY.ARPA>
TO:<@HOSTB.ARPA:USERC@HOSTD.ARPA>.
This command causes its forward-path argument to be appended
to the forward-path buffer.
If service extensions were negotiated, the MAIL command may also
carry parameters associated with a particular service extension.
Syntax: RCPT TO:<forward-path> [ <rcpt-parameters> ]
4.1.1.4 DATA (DATA)
The receiver treats the lines (strings ending in CRLF sequences)
following the command as mail data from the sender. This command
causes the mail data from this command to be appended to the mail
data buffer. The mail data may contain any of the 128 ASCII
character codes.
SMTP is defined in terms of sending messages consisting of lines of
text. Lines are strictly defined as ending in ASCII CR LF sequences.
Systems that use other line delimiting mechanisms internally MUST
convert to CR LF sequences before transmitting mail with unextended
SMTP or with any SMTP service extension on the standards track as of
the time of this writing.
The mail data is terminated by a line containing only a period, that
is the character sequence "<CRLF>.<CRLF>" (see Section 4.6.2 on
Transparency). This is the end of mail data indication.
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 that the server
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
completely the receiver must send a failure reply.
When the SMTP server accepts a message either for relaying or for
final delivery it inserts a trace record (also referred to
interchangabily 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, and the identity of the host that
received the message (and that 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.
4.1.1.5 RESET (RSET)
This command specifies that the current mail transaction is to be
aborted. Any stored sender, recipients, and mail data must be
discarded, and all buffers and state tables cleared. The receiver
must send an OK reply. A reset command may be issued by the client
at any time. It is effectively equivalent to a NOOP if issued
immediately after EHLO or HELO, or before either of those commands
have been issued. In other situations, it restores the state to
that immediately after the most recent EHLO or HELO. 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, below).
4.1.1.6 VERIFY (VRFY)
This command asks the receiver to confirm that the argument
identifies a user. If it is a user name, the full name of
the user (if known) and the fully specified mailbox are
returned.
This command has no effect on any of the reverse-path
buffer, the forward-path buffer, or the mail data buffer.
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. The full name of the users (if
known) and the fully specified mailboxes are returned in a
multiline reply.
This command has no effect on any of the reverse-path
buffer, the forward-path buffer, or the mail data buffer.
4.1.1.8 HELP (HELP)
This command causes the receiver to send helpful information
to the sender of the HELP command. 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 any of the reverse-path
buffer, the forward-path buffer, or the mail data buffer.
SMTP servers SHOULD support HELP even if the form with an argument
is not supported.
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 any of the reverse-path
buffer, the forward-path buffer, or the mail data buffer.
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 send a QUIT command and 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 act as if a
RSET command had been received (cancelling any pending
transaction, but not undoing any previously completed transaction)
and the client MUST act as if the command or transaction in
progress had received a temporary error (4xx).
4.1.1.11 TURN (TURN)
This command, described in RFC 821, raises important security
issues (described in RFC 1123). Its use is deprecated; SMTP
systems SHOULD NOT use it unless the server can authenticate the
client.
4.1.2. LOWER-LEVEL SYNTAX
The syntax of the argument fields of the above commands (using BNF
notation where applicable) is given below. The "..." notation
indicates that a field may be repeated one or more times.
<reverse-path> ::= <path> | "<>"
<forward-path> ::= <path>
<path> ::= "<" [ <a-d-l> ":" ] <mailbox> ">"
<a-d-l> ::= <at-domain> | <at-domain> "," <a-d-l>
<at-domain> ::= "@" <domain>
<mail-parameters> ::= <<<>>
<rcpt-parameters> ::= <<<>>
domain = sub-domain 1*("." sub-domain) | domain-literal
sub-domain = let-dig *(ldh-str)
domain-literal = "[" IP-address-literal "]"
IP-address-literal = snum 3*("." snum)
snum = one, two, or three digits representing a decimal
integer value in the range 0 through 255
let-dig = Alpha / Digit
ldh-str = *( Alpha / Digit / "-" ) 1*(let-dig)
Alpha = ASCII character in the range A-Z or a-z. As specified in
the domain name system definition [RFC-DNS], case is not
significant in domain strings.
Digit = 0 - 9
<mailbox> ::= <local-part> "@" <domain>
<local-part> ::= <dot-string> | <quoted-string>
While the definition for <local-part> above 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.
Systems MUST NOT define mailboxes in such a way as to require the use
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 FROM or RCPT TO commands or other
commands that require mailbox names.
<<?>> <string> ::= <char> | <char> <string>
<<?>> <quoted-string> ::= """ <qtext> """
<<?>> <qtext> ::= "\" <x> | "\" <x> <qtext> | <q> | <q> <qtext>
<char> ::= <c> | "\" <x>
<number> ::= <d> | <d> <number>
<CRLF> ::= <CR> <LF>
<CR> ::= the carriage return character (ASCII code 13)
<LF> ::= the line feed character (ASCII code 10)
<SP> ::= the space character (ASCII code 32)
<snum> ::= one, two, or three digits representing a decimal
integer value in the range 0 through 255
<a> ::= any one of the 52 alphabetic characters A through Z
in upper case and a through z in lower case
<c> ::= any one of the 128 ASCII characters, but not any
<special> or <SP>
<d> ::= any one of the ten digits 0 through 9
<q> ::= any one of the 128 ASCII characters except <CR>,
<LF>, quote ("), or backslash (\)
<x> ::= any one of the 128 ASCII characters (no exceptions)
<special> ::= "<" | ">" | "(" | ")" | "[" | "]" | "\" | "."
| "," | ";" | ":" | "@" """ | the control
characters (ASCII codes 0 through 31 inclusive and
127)
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" could be used to indicate a single nine character
user field with comma being the fourth character of the field.
Hosts are generally known by names which are translated to addresses
in each host. Note that the name elements of domains must be
resolvable in the Internet domain system. Local aliases or nicknames
MUST NOT be used.
Characters outside the set of specials, alphas, digits, and hyphen
are prohibited by the domain name system definition and MUST NOT
appear in domain names. In particular, the underscore character is
not permitted.
Sometimes a host is not known to the translation function and
communication is blocked. To bypass this barrier a numeric form is
also allowed for host "names". This form uses four or more small
decimal integers separated by dots and enclosed by brackets, e.g.,
"[123.255.37.2]", which indicates an Internet Address in
sequence-of-octets form.
The earlier escape form that uses a decimal integer prefixed by a
pound sign, "#", indicating the number is the address of the host, is
deprecated and MUST NOT be used.
The time stamp line and the return path line are formally defined as
follows:
<return-path-line> ::= "Return-Path:" <SP><reverse-path><CRLF>
<time-stamp-line> ::= "Received:" <SP> <stamp> <CRLF>
<stamp> ::= <from-domain> <by-domain> <opt-info> ";"
<daytime>
<from-domain> ::= "FROM" <SP> <domain> <SP>
<by-domain> ::= "BY" <SP> <domain> <SP>
<opt-info> ::= [<via>] [<with>] [<id>] [<for>]
<via> ::= "VIA" <SP> <link> <SP>
<with> ::= "WITH" <SP> <protocol> <SP>
<id> ::= "ID" <SP> <string> <SP>
<for> ::= "FOR" <SP> <path> <SP>
<<>>FOR and <link> need to be nailed down.
<link> ::= The standard names for links are registered with
the Internet Assigned Numbers Authority (IANA).
<protocol> ::= The standard names for protocols are
registered with the Internet Assigned Numbers Authority
(IANA).
<daytime> ::= <SP> <date> <SP> <time>
<date> ::= <dd> <SP> <mon> <SP> <yyyy>
Note that the earlier form, which permits
two-digit years, is deprecated. SMTP systems
SHOULD use four-digit years.
<time> ::= <hh> ":" <mm> ":" <ss> <SP> <zone>
<dd> ::= the one or two decimal integer day of the month in
the range 1 to 31.
<mon> ::= "JAN" | "FEB" | "MAR" | "APR" | "MAY" | "JUN" |
"JUL" | "AUG" | "SEP" | "OCT" | "NOV" | "DEC"
<yyyy> ::= the four decimal integer year in the range 0000 to
9999.
<hh> ::= the two decimal integer hour of the day in the
range 00 to 24.
<mm> ::= the two decimal integer minute of the hour in the
range 00 to 59.
<ss> ::= the two decimal integer second of the minute in the
range 00 to 59.
<zone> ::= A four-digit time zone offset, such as -0600 for US
Eastern Standard Time. This may be supplemented by a
time zone name in parentheses, e.g., "-0800 (PDT)". See
??? for additional discussion.
Note that there is no default; time zone information
is required and MUST be supplied.
-------------------------------------------------------------
Return Path Example
Return-Path: <@CHARLIE.ARPA,@BAKER.ARPA:JOE@ABLE.ARPA>
Example 9
-------------------------------------------------------------
-------------------------------------------------------------
Time Stamp Line Example
Received: FROM ABC.ARPA BY XYZ.ARPA ; 22 OCT 81 09:23:59 PDT
Received: from ABC.ARPA by XYZ.ARPA via TELENET with X25
id M12345 for Smith@PDQ.ARPA ; 22 OCT 81 09:23:59 PDT
Example 10
-------------------------------------------------------------
4.1.3. Order of commands
There are restrictions on the order in which these commands may
be used.
A session that is to contain mail transactions MUST first be
initialized by the use of the HELO or EHLO command. An SMTP server
SHOULD accept commands for non-mail transactions (e.g., VRFY or EXPN)
without this initialization.
HELO or EHLO commands MAY be issued by a client later in the session.
If either is issued after the session begins, the SMTP server MUST
clear all buffers and state as if an RSET command had been issued.
In other words, the sequence of RSET followed immediately by HELO is
redundant, but not harmful other than in the performance cost of
executing unnecessary commands.
If the HELO or EHLO commands are 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 as it was in before the HELO or EHLO were received.
RFC 1123 contains a discussion of arguments to HELO and conditions
under which the HELO command can be rejected. In particular, HELO
(or EHLO) MUST NOT be rejected because the client's putative name
does not match some criteria established by the server (e.g.,
verification of reverse DNS mapping).
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 (i.e., not return a 503 code)
even if no HELO or EHLO command has yet been received; clients SHOULD
open a session with HELO or EHLO before sending these commands.
If the above rules are followed, the example in RFC 821 that shows
"550 access denied to you" in response to an EXPN command is
essentially meaningless unless a HELO or EHLO command preceeds the
EXPN or the denial of access is based on the client's IP address.
The MAIL, SEND, SOML, or SAML commands begin a mail transaction.
Once started a mail transaction consists of one of the transaction
beginning commands, one or more RCPT commands, and a DATA command, in
that order. A mail transaction may be aborted by the RSET command.
There may be zero or more transactions in a session.
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 can not be used at any other time in a session, but may be
used by the client SMTP to request connection-closing even if no
session-opening command has been sent and accepted.
4.1.4 Private-use commands
Commands starting in "X" may be used by bilateral agreement
between the client (sending) and server (receiving) SMTPs.
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 be documented in published RFCs and be at
least candidates for standardization.
4.2. SMTP REPLIES
Replies to SMTP commands are devised 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 made explicit in
Section 4.3 on Sequencing and Section 4.5 containing State Diagrams.
An SMTP reply consists of a three digit number (transmitted as three
alphanumeric characters) followed by some text. The number is
intended for use by automata to determine what state to enter next;
the text is meant for the human user. It is intended that 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. In particular, 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 Appendix E. 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 Appendix E). Only the
EXPN and HELP commands are expected to result in multiline replies in
normal circumstances, however multiline replies are allowed for any
command.
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.
A client SMTP MUST determine its actions only by the reply
code, not by the text (except for 251 and 551 replies); any
text, including no text at all, must be acceptable. The space
(blank) following the reply code is considered part of the
text. Whenever possible, a sender-SMTP SHOULD test only the
first digit of the reply code.
4.2.1. 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 4.2.3)
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>
252 Cannot VRFY user, but will accept message and attempt delivery
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]
451 Requested action aborted: error in processing
551 User not local; please try <forward-path>
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
4.2.2. NUMERIC ORDER LIST OF REPLY CODES
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>
252 Cannot VRFY user, but will accept message and attempt delivery
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
503 Bad sequence of commands
504 Command parameter not implemented
550 Requested action not taken: mailbox unavailable
[E.g., mailbox not found, no access]
551 User not local; please try <forward-path>
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
4.2.3. 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.
4.2.4 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 responsibity 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 RFC 1123, or
+ 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 FROM 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 further attempt to deliver that message.
The SMTP client retains responsibility for delivery of that
message. The sending user should be able to interpret the
return of a transient or permanent failure status as a
non-delivery indication.
4.3. SEQUENCING OF COMMANDS AND REPLIES
4.3.1 Sequencing overview
The communication between the sender and receiver is intended to be
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 (i.e., the
fully-qualified primary domain name) of the server host as the first
word following the reply code. When the host has no name, the IP
address should be used, in bracketed dotted-octet format, e.g.,
[10.0.0.6].
For example,
220 <SP> USC-ISIF.ARPA <SP> Service ready <CRLF>
The table below lists alternative success and failure replies for
each command. These must 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.
COMMAND-REPLY SEQUENCES
Each command is listed with its usual possible replies. The prefixes
used before the possible replies are "P" for preliminary (not used in
SMTP), "I" for intermediate, "S" for success, "F" for failure, and
"E" for error. The 421 reply (service not available, closing
transmission channel) may be given to any command if the
SMTP-receiver knows it must shut down. 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. 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. This listing forms the basis for the State Diagrams in
Section 4.5.
CONNECTION ESTABLISHMENT
S: 220
F: 421
HELO
S: 250
E: 500, 501, 504, 421
MAIL
S: 250
F: 552, 451, 452
E: 500, 501, 421
RCPT
S: 250, 251 (but see section <<<>>> for discussion of 251)
F: 550, 551, 552, 553, 450, 451, 452
E: 500, 501, 503, 421
DATA
I: 354 -> data -> S: 250
F: 552, 554, 451, 452
F: 451, 554
E: 500, 501, 503, 421
RSET
S: 250
E: 500, 501, 504, 421
SEND
S: 250
F: 552, 451, 452
E: 500, 501, 502, 421
SOML
S: 250
F: 552, 451, 452
E: 500, 501, 502, 421
SAML
S: 250
F: 552, 451, 452
E: 500, 501, 502, 421
VRFY
S: 250, 251
F: 550, 551, 553
E: 500, 501, 502, 504, 421
EXPN
S: 250
F: 550
E: 500, 501, 502, 504, 421
HELP
S: 211, 214
E: 500, 501, 502, 504, 421
NOOP
S: 250
E: 500, 421
QUIT
S: 221
E: 500
TURN
S: 250
F: 502
E: 500, 503
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 body, as discussed under
the DATA command in section 4.1.1.4.
This line must be structured as follows:
* The FROM field SHOULD contain both (1) the name of the
source host as presented in the EHLO or HELO command and (2) a
domain 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.
An Internet mail program MUST NOT change a Received: line that was
previously added to the message header.
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. If it is
desired to also use a time zone name, it should be included in a
commment.
When the 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 leaves the
SMTP world. Normally, this would mean it has been delivered to the
destination user, 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 be different from
the actual sender's mailbox, for example, if error responses are to
be delivered a special error handling mailbox rather than to that of
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 preceding two paragraphs imply 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 header and body
[RFC822]. See Example 8.
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. However any
further (forwarding, gateway, or relay) systems MAY remove the return
path and rebuild the MAIL FROM 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. If a message that contains
more than one Return-path header is received, only the first
Return-path header line in the message header is valid. A message
header processor SHOULD discard or, if necessary just ignore, any
Return-path headers following the first one.
The primary intent of the Return-path is that it designates the
address to which messages indicating non-delivery or other mail
system failures at to be sent. For this to be unambigious, exactly
one return path should be present when the message is delivered.
Systems using RFC 822 syntax with non-SMTP transports SHOULD preserve
the intent of having an unambiguous address, associated with the
transport envelope, to which to send error reports (e.g.,
non-delivery messages).
Historical note: Text in RFC 822 that appears to contradict the use
of Return-path (or the envelope MAIL FROM address) as the destination
of error messages is not applicable on the Internet. The MAIL FROM
address (as copied into the Return-path) MUST be used as the target
of any mail containing delivery error messages.
In particular,
(i) 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.
(ii) 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 MAIL FROM part of the SMTP envelope.
Special mention is needed of the response and further action required
when the processing following the end of mail data indication is
partially successful. This could arise if after accepting several
recipients and the mail data, the SMTP server finds that the mail
data can be successfully delivered to some of the recipients, but it
cannot be to others (for example, due to mailbox space allocation
problems). In such a situation, the response to the DATA command
must be an OK reply. But, the SMTP server must compose and send an
"undeliverable mail" notification message to the originator of the
message. Either a single notification which lists all of the
recipients that failed to get the message, or separate notification
messages must be sent for each failed recipient (see Example 7). All
undeliverable mail notification messages are sent using the MAIL
command (even if they result from processing a SEND, SOML, or SAML
command).
-------------------------------------------------------------
Example of Return Path and Received Time Stamps
Return-Path: <@GHI.ARPA,@DEF.ARPA,@ABC.ARPA:JOE@ABC.ARPA>
Received: from GHI.ARPA by JKL.ARPA ; 27 Oct 81 15:27:39 PST
Received: from DEF.ARPA by GHI.ARPA ; 27 Oct 81 15:15:13 PST
Received: from ABC.ARPA by DEF.ARPA ; 27 Oct 81 15:01:59 PST
Date: 27 Oct 81 15:01:01 PST
From: JOE@ABC.ARPA
Subject: Improved Mailing System Installed
To: SAM@JKL.ARPA
This is to inform you that ...
Example 8
-------------------------------------------------------------
4.5. STATE DIAGRAMS
Following are state diagrams for a simple-minded SMTP
implementation. Only the first digit of the reply codes is used.
There is one state diagram for each group of SMTP commands. The
command groupings were determined by constructing a model for each
command and then collecting together the commands with
structurally identical models.
For each command there are three possible outcomes: "success"
(S), "failure" (F), and "error" (E). In the state diagrams below
we use the symbol B for "begin", and the symbol W for "wait for
reply".
First, the diagram that represents most of the SMTP commands:
1,3 +---+
----------->| E |
| +---+
|
+---+ cmd +---+ 2 +---+
| B |---------->| W |---------->| S |
+---+ +---+ +---+
|
| 4,5 +---+
----------->| F |
+---+
This diagram models the commands:
HELO, EHLO, MAIL, RCPT, RSET, SEND, SOML, SAML, VRFY, EXPN,
HELP, NOOP, QUIT, TURN.
A more complex diagram models the DATA command:
+---+ DATA +---+ 1,2 +---+
| B |---------->| W |-------------------->| E |
+---+ +---+ ------------>+---+
3| |4,5 |
| | |
-------------- ----- |
| | | +---+
| ---------- -------->| S |
| | | | +---+
| | ------------
| | | |
V 1,3| |2 |
+---+ data +---+ --------------->+---+
| |---------->| W | | F |
+---+ +---+-------------------->+---+
4,5
Note that the "data" here are a series of lines sent from the
sender to the receiver with no response expected until the last
line is sent.
4.6. DETAILS
4.6.1. MINIMUM IMPLEMENTATION
In order to make SMTP workable, the following minimum
implementation is required for all receivers:
COMMANDS -- HELO
VRFY
MAIL
RCPT
DATA
RSET
NOOP
QUIT
Any system that includes an SMTP server that supports RCPT MUST
support the reserved mailbox "Postmaster" as a case-insensitive
mailbox name.
4.6.2. 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.
1. 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.
2. 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 the end of mail. 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 format effectors 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.
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, or that store data in records rather than
strings. If such transforms are necessary, they must be
reversible -- especially if such transforms are applied to
mail being relayed.
4.6.3. SIZES AND TIMEOUTS
There are several objects that have required minimum maximum
sizes. That is, every implementation must be able to receive
objects of at least these sizes, but must not send objects
larger than these sizes.
****************************************************
* *
* TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION *
* TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH *
* OF THESE OBJECTS SHOULD BE USED. *
* *
****************************************************
user
The maximum total length of a user name is 64 characters.
domain
The maximum total length of a domain name or number is 64
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.
reply line
The maximum total length of a reply line including the
reply code and the <CRLF> is 512 characters.
text line
The maximum total length of a text line including the
<CRLF> is 1000 characters (but not counting the leading
dot duplicated for transparency). This number may be increased by
the use of SMTP Service Extensions.
message body
The maximum total length of a message body (including any message
headers) MUST BE at least 64K octets. Especially 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 maximum total number of recipients that must be
buffered is 100 recipients.
****************************************************
* *
* TO THE MAXIMUM EXTENT POSSIBLE, IMPLEMENTATION *
* TECHNIQUES WHICH IMPOSE NO LIMITS ON THE LENGTH *
* OF THESE OBJECTS SHOULD BE USED. *
* *
****************************************************
Errors due to exceeding these limits may be reported by using
the reply codes, for example:
500 Line too long.
501 Path too long
552 Too many recipients.
552 Too much mail data.
An SMTP client should provide timeouts for all commands. Minimum
values SHOULD be as follows:
o 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 will accept a
TCP connection but delay delivery of the 220 message until
their system load will permit more mail to be processed.
o MAIL Command: 5 minutes
o RCPT Command: 5 minutes
A longer timeout would be required if processing of
mailing lists and aliases were not deferred until after
the message was accepted.
o DATA Initiation: 2 minutes
This is while awaiting the "354 Start Input" reply to a
DATA command.
o Data Block: 3 minutes
This is while awaiting the completion of each TCP SEND
call transmitting a chunk of data.
o 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, since the message has been
successfully sent.
An SMTP server SHOULD have a timeout of at least 5 minutes
while it is awaiting the next command from the sender.
4.6.4 Queuing Strategies
The common structure of a host SMTP implementation includes
user mailboxes, one or more areas for queueing 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 queueing strategy MUST include:
o Timeouts on all activities on a per-command basis
o Never sending error messages in response to error messages.
4.6.4.1 Sending Strategy
The general model of 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 sender SHOULD keep a list of hosts it cannot reach and
corresponding connection timeouts, rather than just retrying queued
mail items.
DISCUSSION:
Experience suggests that failures are typically
transient (the target system has crashed), favoring a
policy of two connection attempts in the first hour the
message is in the queue, and then backing off to once
every two or three hours.
The SMTP client can shorten the queueing delay by
cooperation with the SMTP server. In particular, if
mail is received from a particular address, it is good
evidence that any mail queued for that host can now be
sent.
The strategy may be further modified as a result of
multiple addresses per host (see Section 5.3.4), to
optimize delivery time vs. resource usage.
an SMTP client may have a large queue of messages for
each unavailable destination host, and if it retried
all these messages in every retry cycle, there would be
excessive Internet overhead and the daemon would be
blocked for a long period. Note that an SMTP can
generally determine that a delivery attempt has failed
only after a timeout of a minute or more; a one minute
timeout per connection will result in a very large
delay if it is repeated for dozens or even hundreds of
queued messages.
When the same message is to be delivered to several users on
the same host, only one copy of the message SHOULD be
transmitted. That is, the SMTP client should use the
command sequence: RCPT, RCPT,... RCPT, DATA instead of the
sequence: RCPT, DATA, RCPT, DATA,... RCPT, DATA.
Implementation of this efficiency feature is strongly urged.
Similarly, the SMTP client MAY support multiple concurrent
outgoing mail transactions to achieve timely delivery.
However, some limit SHOULD be imposed to protect the host
from devoting all its resources to mail.
4.6.4.2 Receiving strategy
The SMTP server SHOULD attempt to keep a pending listen on
the SMTP port at all times. This will require the support
of multiple incoming TCP connections for SMTP. Some limit
MAY be imposed.
IMPLEMENTATION:
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.
5. Problem detection and handling
5.1 Replies by email
<<>>
5.2 Loop detection
Simple counting of the number of Received lines in a message has not
proven to be a desirable method of detecting loops in mail systems,
and SMTP servers SHOULD NOT use that technique. Loop detection by
examination of Received fields for the domain name or other signature
of the SMTP server making the check is effective and MAY be used by
SMTP servers.
6. Security Considerations
6.1 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 security lies only in
end-to-end methods involving the message bodies, e.g., those
that can be provided in the MOSS framework [RFC-MOSS].
A corollary to this is that efforts to make it more difficult
for users to set envelope MAIL FROM and header "From" fields
to point to valid addresses other than their own are largely
misguided: they do not prevent any would-be mail spoofer from
doing so, and do 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. On the other hand, 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 can be generated
correctly.
This specification does not further address the security 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.
6.2 "Blind" copies.
Addresses may appear in the RCPT TO commands to an SMTP server
that do not appear in the message headers for a number of
reasons. The two most common of these 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". In order to avoid defeating some of the purpose of
these mechanisms, SMTP clients and servers SHOULD NOT copy the
RCPT TO command arguments into the headers, even 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 TO command.
More generally, while there are often similarities, there is no
inherent relationship between either "reverse" (MAIL FROM, SAML FROM,
etc.) or "forward" (RCPT TO) 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 and SHOULD NOT be used.
See also section ##2.2.4.
7. REFERENCES
[1] ASCII
ASCII, "USA Code for Information Interchange", United States of
America Standards Institute, X3.4, 1968.
[RFC822]
Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", RFC 822, Department of Electrical Engineering,
University of Delaware, August 1982.
[3] TCP
Postel, J., ed., "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", RFC 793, USC/Information Sciences
Institute, NTIS AD Number A111091, September 1981. Also in:
Feinler, E. and J. Postel, eds., "Internet Protocol Transition
Workbook", SRI International, Menlo Park, California, March 1982.
[HEADER-PEOPLE]
[RFC-DNS] P. Mockapetris, "Domain names - implementation and
specification", RFC 1035 and P. Mockapetris, "Domain names -
concepts and facilities", RFC 1034. (STD 13)
[RFC 974] C. Partridge, "Mail routing and the domain system",
01/01/1986
[SMTPEX] J. Klensin, N. Freed, M. Rose, E. Stefferud, D.
Crocker, "SMTP Service Extensions", RFC-1869, 11/06/1995.
[RFC-1123] R. Braden, "Requirements for Internet hosts -
application and support", 10/01/1989
[RFC-MOSS] S. Crocker, N. Freed, J. Galvin, S. Murphy, "MIME Object
Security Services", RFC 1848, 10/03/1995.
[RFC-POP2] M. Butler, D. Chase, J. Goldberger, J. Postel, J.
Reynolds, "Post Office Protocol - version 2", RFC 937,
02/01/1985
[RFC-POP3] J. Myers, M. Rose, "Post Office Protocol - Version 3",
RFC 1725, 11/23/1994.
[RFC-IMAP4] 1730 PS M. Crispin, "Internet Message Access Protocol
- Version 4", RFC 1730, 12/20/1994.
8. Editor's Addresses
John C. Klensin
MCI Data Services
800 Boylston St, 7th floor
Boston, MA 02199
USA
Email: Klensin@mci.net
Phone: +1 617 859 1011
Fax: +1 617 859 1011
????
9. Acknowledgements
<<to be supplied>>
APPENDIX A
TCP Transport service
The Transmission Control Protocol [3] is used in the Internet, and in
any network following the Internet standards for internetwork protocols.
Connection Establishment
The SMTP transmission channel is a TCP connection established
between the sender process port U and the receiver process port
L. This single full duplex connection is used as the
transmission channel. This protocol is assigned the service
port 25 (31 octal), that is L=25.
Data Transfer
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.
APPENDIX 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:
(i) each recipient addresses 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 (see section 2.15
below).
(ii) the return address in the MAIL command should be derived from
the system's identity for the submitting (local) user. That return
address 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.)
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.
APPENDIX E
Theory of Reply Codes
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 will be able to
determine its next action (proceed as planned, redo, retrench,
etc.) by simply 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, reserving the third digit 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: SMTP does not have any commands that allow this
type of reply, and so does not have the 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.
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- SMTPs) 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 if
a reply fits into the 4yz or the 5yz category (see below)
is that replies are 4yz if they can be repeated without
any change in command form or in properties of the sender
or receiver. (E.g., 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 don't 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 as yet.
x4z Unspecified as yet.
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 response 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>.
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 sender in the reply "text". The sender will know
these cases from the current context.
APPENDIX F
Scenarios
This section presents complete scenarios of several types of SMTP
sessions.
A Typical SMTP Transaction Scenario
This SMTP example shows mail sent by Smith at host USC-ISIF, to
Jones, Green, and Brown at host BBN-UNIX. Here we assume that
host USC-ISIF contacts host BBN-UNIX directly. The mail is
accepted for Jones and Brown. Green does not have a mailbox at
host BBN-UNIX.
-------------------------------------------------------------
R: 220 BBN-UNIX.ARPA Simple Mail Transfer Service Ready
S: HELO USC-ISIF.ARPA
R: 250 BBN-UNIX.ARPA
S: MAIL FROM:<Smith@USC-ISIF.ARPA>
R: 250 OK
S: RCPT TO:<Jones@BBN-UNIX.ARPA>
R: 250 OK
S: RCPT TO:<Green@BBN-UNIX.ARPA>
R: 550 No such user here
S: RCPT TO:<Brown@BBN-UNIX.ARPA>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 BBN-UNIX.ARPA Service closing transmission channel
Scenario 1
-------------------------------------------------------------
Aborted SMTP Transaction Scenario
-------------------------------------------------------------
R: 220 MIT-Multics.ARPA Simple Mail Transfer Service Ready
S: HELO ISI-VAXA.ARPA
R: 250 MIT-Multics.ARPA
S: MAIL FROM:<Smith@ISI-VAXA.ARPA>
R: 250 OK
S: RCPT TO:<Jones@MIT-Multics.ARPA>
R: 250 OK
S: RCPT TO:<Green@MIT-Multics.ARPA>
R: 550 No such user here
S: RSET
R: 250 OK
S: QUIT
R: 221 MIT-Multics.ARPA Service closing transmission channel
Scenario 2
-------------------------------------------------------------
Relayed Mail Scenario
-------------------------------------------------------------
Step 1 -- Source Host to Relay Host
R: 220 USC-ISIE.ARPA Simple Mail Transfer Service Ready
S: HELO MIT-AI.ARPA
R: 250 USC-ISIE.ARPA
S: MAIL FROM:<JQP@MIT-AI.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:Jones@BBN-VAX.ARPA>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Date: 2 Nov 81 22:33:44
S: From: John Q. Public <JQP@MIT-AI.ARPA>
S: Subject: The Next Meeting of the Board
S: To: Jones@BBN-Vax.ARPA
S:
S: Bill:
S: The next meeting of the board of directors will be
S: on Tuesday.
S: John.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISIE.ARPA Service closing transmission channel
Step 2 -- Relay Host to Destination Host
R: 220 BBN-VAX.ARPA Simple Mail Transfer Service Ready
S: HELO USC-ISIE.ARPA
R: 250 BBN-VAX.ARPA
S: MAIL FROM:<@USC-ISIE.ARPA:JQP@MIT-AI.ARPA>
R: 250 OK
S: RCPT TO:<Jones@BBN-VAX.ARPA>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Received: from MIT-AI.ARPA by USC-ISIE.ARPA ;
2 Nov 81 22:40:10 UT
S: Date: 2 Nov 81 22:33:44
S: From: John Q. Public <JQP@MIT-AI.ARPA>
S: Subject: The Next Meeting of the Board
S: To: Jones@BBN-Vax.ARPA
S:
S: Bill:
S: The next meeting of the board of directors will be
S: on Tuesday.
S: John.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISIE.ARPA Service closing transmission channel
Scenario 3
-------------------------------------------------------------
Verifying and Sending Scenario
-------------------------------------------------------------
R: 220 SU-SCORE.ARPA Simple Mail Transfer Service Ready
S: HELO MIT-MC.ARPA
R: 250 SU-SCORE.ARPA
S: VRFY Crispin
R: 250 Mark Crispin <Admin.MRC@SU-SCORE.ARPA>
S: SEND FROM:<EAK@MIT-MC.ARPA>
R: 250 OK
S: RCPT TO:<Admin.MRC@SU-SCORE.ARPA>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 SU-SCORE.ARPA Service closing transmission channel
Scenario 4
-------------------------------------------------------------
Mailing List Scenario
First each of two mailing lists are expanded in separate sessions
with different hosts. Then the message is sent to everyone that
appeared on either list (but no duplicates) via a relay host.
-------------------------------------------------------------
Step 1 -- Expanding the First List
R: 220 MIT-AI.ARPA Simple Mail Transfer Service Ready
S: HELO SU-SCORE.ARPA
R: 250 MIT-AI.ARPA
S: EXPN Example-People
R: 250-<ABC@MIT-MC.ARPA>
R: 250-Fred Fonebone <Fonebone@USC-ISIQ.ARPA>
R: 250-Xenon Y. Zither <XYZ@MIT-AI.ARPA>
R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
R: 250-<joe@foo-unix.ARPA>
R: 250 <xyz@bar-unix.ARPA>
S: QUIT
R: 221 MIT-AI.ARPA Service closing transmission channel
Step 2 -- Expanding the Second List
R: 220 MIT-MC.ARPA Simple Mail Transfer Service Ready
S: HELO SU-SCORE.ARPA
R: 250 MIT-MC.ARPA
S: EXPN Interested-Parties
R: 250-Al Calico <ABC@MIT-MC.ARPA>
R: 250-<XYZ@MIT-AI.ARPA>
R: 250-Quincy Smith <@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
R: 250-<fred@BBN-UNIX.ARPA>
R: 250 <xyz@bar-unix.ARPA>
S: QUIT
R: 221 MIT-MC.ARPA Service closing transmission channel
Step 3 -- Mailing to All via a Relay Host
R: 220 USC-ISIE.ARPA Simple Mail Transfer Service Ready
S: HELO SU-SCORE.ARPA
R: 250 USC-ISIE.ARPA
S: MAIL FROM:<Account.Person@SU-SCORE.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:ABC@MIT-MC.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:Fonebone@USC-ISIQA.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:XYZ@MIT-AI.ARPA>
R: 250 OK
S: RCPT
TO:<@USC-ISIE.ARPA,@USC-ISIF.ARPA:Q-Smith@ISI-VAXA.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:joe@FOO-UNIX.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:xyz@BAR-UNIX.ARPA>
R: 250 OK
S: RCPT TO:<@USC-ISIE.ARPA:fred@BBN-UNIX.ARPA>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 USC-ISIE.ARPA Service closing transmission channel
Scenario 7
-------------------------------------------------------------
Too Many Recipients Scenario
-------------------------------------------------------------
R: 220 BERKELEY.ARPA Simple Mail Transfer Service Ready
S: HELO USC-ISIF.ARPA
R: 250 BERKELEY.ARPA
S: MAIL FROM:<Postel@USC-ISIF.ARPA>
R: 250 OK
S: RCPT TO:<fabry@BERKELEY.ARPA>
R: 250 OK
S: RCPT TO:<eric@BERKELEY.ARPA>
R: 552 Recipient storage full, try again in another transaction
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: MAIL FROM:<Postel@USC-ISIF.ARPA>
R: 250 OK
S: RCPT TO:<eric@BERKELEY.ARPA>
R: 250 OK
S: DATA
R: 354 Start mail input; end with <CRLF>.<CRLF>
S: Blah blah blah...
S: ...etc. etc. etc.
S: .
R: 250 OK
S: QUIT
R: 221 BERKELEY.ARPA Service closing transmission channel
Scenario 10
-------------------------------------------------------------
Note that a real implementation must handle many recipients as
specified in Section 4.5.3.
APPENDIX G 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, e.g., 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.
APPENDIX H GLOSSARY
ASCII
American Standard Code for Information Interchange [1].
command
A request for a mail service action sent by the SMTP client to the
SMTP server.
domain
The hierarchially structured global character string address of a
host computer in the mail system.
end of mail data indication
A special sequence of characters that indicates the end of the
mail data. In particular, the five characters carriage return,
line feed, period, carriage return, line feed, in that order.
host
A computer in the internetwork environment on which mailboxes or
SMTP processes reside.
line
A a sequence of ASCII characters ending with a <CRLF>.
mail data
A sequence of ASCII characters of arbitrary length, which conforms
to the standard set in the Standard for the Format of ARPA
Internet Text Messages (RFC 822 [RFC822]).
mailbox
A character string (address) which identifies a user to whom mail
is to be sent. Mailbox normally consists of the host and user
specifications. The standard mailbox naming convention is defined
to be "user@domain". Additionally, the "container" in which mail
is stored.
SMTP server process
A process which transfers mail in cooperation with an SMTP client
process. It waits for a connection to be established via the
transport service. It receives SMTP commands from the
SMTP client, sends replies, and performs the specified operations.
reply
A 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 completion code
(including error codes) followed by a text string. The codes are
for use by programs and the text is usually intended for human
users.
SMTP client process
A process which transfers mail in cooperation with an SMTP server
process. A local language may be used in the user interface
command/reply dialogue. The SMTP client initiates the transport
service connection. It initiates SMTP commands, receives replies,
and governs the transfer of mail.
session
The set of exchanges that occur while the transmission channel is
open.
transaction
The set of exchanges required for one message to be transmitted
for one or more recipients.
transmission channel
A full-duplex communication path between an SMTP client and a
SMTP server for the exchange of commands, replies, and mail
text.
transport service
Any reliable stream-oriented data communication services. For
example, NCP, TCP, NITS.
user
A human being (or a process on behalf of a human being) wishing to
obtain mail transfer service. In addition, a recipient of
computer mail.
word
A sequence of printing characters.
<CRLF>
The characters carriage return and line feed (in that order).
<SP>
The space character.
--------------------------------------------------
What to do with the following... ??
??.??.?? Time semantics
The Date specified in the RFC 822 header is normally presumed
to be the date and time at which the user completed
composition of the message. As such, it SHOULD be supplied by
the message composing system. An SMTP server MUST NOT alter
this date if it appears in a message received by it for
transmission or relaying.
In environments in which mail composition agents queue mail
locally before transmitting it to a sending SMTP server (or
the other component of a split-MUA arrangement), the RFC 822
date SHOULD reflect the time at which message composition was
completed, not the time of transfer to the MTA.
In environments in which Mail User Agent/ Mail Transfer Agent
distinctions are made, SMTP servers normally SHOULD insert a
Received field to reflect the receipt of the message for
transmission from the MUA.
APPENDIX 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.
**Please Review This Text**.
(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.2 Loose ends
(i) Material in RFC1123, section 5.2.6, not yet fully incorporated.
(ii) The following text needs to go in somewhere in some form.
Please note that, when RFC 822 format is being used, the mail data
includes the memo header items such as Date, Subject, To, Cc, From
[RFC822]. Server SMTP systems SHOULD NOT reject messages based on
perceived defects in the RFC 822 or MIME [MIME] message header or
message body. In particular, they MUST NOT reject messages on the
basis of trying to match numbers of Resent- fields. In particular,
messages MUST NOT be rejected because Resent-to appears without
Resent-from, Resent-date, or both.
(iii) Are 5.3.3 and 5.3.4 of RFC1123 adequately incorporated?
(iv) What needs to be done about RFC1123 5.3.6 and 5.3.7 and where
should it/they go?
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draft-ietf-drums-smtpupd-01
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