RFC2476 - Message Submission

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　　Network Working Group R. Gellens
Request for Comments: 2476 QUALCOMM
Category: Standards Track J. Klensin
MCI
December 1998

Message Submission

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1998). All Rights Reserved.

Table of Contents

1. Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Document Information . . . . . . . . . . . . . . . . . . . 3
2.1. Definitions of Terms Used in this Memo . . . . . . . . . 3
2.2. Conventions Used in this Document . . . . . . . . . . . 4
3. Message Submission . . . . . . . . . . . . . . . . . . . . . 4
3.1. Submission Identification . . . . . . . . . . . . . . . 4
3.2. Message Rejection and Bouncing . . . . . . . . . . . . . 4
3.3. Authorized Submission . . . . . . . . . . . . . . . . . 5
3.4. Enhanced Status Codes . . . . . . . . . . . . . . . . . 6
4. Mandatory Actions . . . . . . . . . . . . . . . . . . . . . 6
4.1. General Submission Rejection Code . . . . . . . . . . . 6
4.2. Ensure All Domains are Fully-Qualified . . . . . . . . 6
5. Recommended Actions . . . . . . . . . . . . . . . . . . . . 7
5.1. Enforce Address Syntax . . . . . . . . . . . . . . . . 7
5.2. Log Errors . . . . . . . . . . . . . . . . . . . . . . . 7
6. Optional Actions . . . . . . . . . . . . . . . . . . . . . 7
6.1. Enforce Submission Rights . . . . . . . . . . . . . . . 7
6.2. Require Authentication . . . . . . . . . . . . . . . . 8
6.3. Enforce Permissions . . . . . . . . . . . . . . . . . . 8
6.4. Check Message Data . . . . . . . . . . . . . . . . . . 8
7. Interaction with SMTP Extensions . . . . . . . . . . . . . . 8
8. Message Modifications . . . . . . . . . . . . . . . . . . . 9
8.1. Add 'Sender' . . . . . . . . . . . . . . . . . . . . . . 9
8.2. Add 'Date' . . . . . . . . . . . . . . . . . . . . . . 10
8.3. Add 'Message-ID' . . . . . . . . . . . . . . . . . . . . 10

8.4. Transfer Encode . . . . . . . . . . . . . . . . . . . . 10
8.5. Sign the Message . . . . . . . . . . . . . . . . . . . . 10
8.6. Encrypt the Message . . . . . . . . . . . . . . . . . . 10
8.7. Resolve Aliases . . . . . . . . . . . . . . . . . . . . 10
8.8. Header Rewriting . . . . . . . . . . . . . . . . . . . 10
9. Security Considerations . . . . . . . . . . . . . . . . . . 11
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 11
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
12. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 14
13. Full Copyright Statement . . . . . . . . . . . . . . . . . 15

1. Abstract

SMTP was defined as a message *transfer* protocol, that is, a means
to route (if needed) and deliver finished (complete) messages.
Message Transfer Agents (MTAs) are not supposed to alter the message
text, except to add 'Received', 'Return-Path', and other header
fields as required by [SMTP-MTA].

However, SMTP is now also widely used as a message *submission*
protocol, that is, a means for message user agents (MUAs) to
introduce new messages into the MTA routing network. The process
which accepts message submissions from MUAs is termed a Message
Submission Agent (MSA).

Messages being submitted are in some cases finished (complete)
messages, and in other cases are unfinished (incomplete) in some
aspect or other. Unfinished messages need to be completed to ensure
they conform to [MESSAGE-FORMAT], and later requirements. For
example, the message may lack a proper 'Date' header field, and
domains might not be fully qualified. In some cases, the MUA may be
unable to generate finished messages (for example, it might not know
its time zone). Even when submitted messages are complete, local
site policy may dictate that the message text be examined or modified
in some way. Such completions or modifications have been shown to
cause harm when performed by downstream MTAs -- that is, MTAs after
the first-hop submission MTA -- and are in general considered to be
outside the province of standardized MTA functionality.

Separating messages into submissions and transfers allows developers
and network administrators to more easily:

* Implement security policies and guard against unauthorized mail
relaying or injection of unsolicited bulk mail

* Implement authenticated submission, including off-site submission
by authorized users such as travelers

* Separate the relevant software code differences, thereby making
each code base more straightforward and allowing for different
programs for relay and submission

* Detect configuration problems with a site's mail clients

* Provide a basis for adding enhanced submission services in the
future

This memo describes a low cost, deterministic means for messages to
be identified as submissions, and specifies what actions are to be
taken by a submission server.

Public comments should be sent to the IETF Submit mailing list,
<ietf-submit@imc.org>. To subscribe, send a message containing
SUBSCRIBE to <ietf-submit-request@imc.org>. Private comments may be
sent to the authors.

2. Document Information

2.1. Definitions of Terms Used in this Memo

Fully-Qualified

Containing or consisting of a domain which can be globally resolved
using the global Domain Name Service; that is, not a local alias or
partial specification.

Message Submission Agent (MSA)

A process which conforms to this specification, which acts as a
submission server to accept messages from MUAs, and either delivers
them or acts as an SMTP client to relay them to an MTA.

Message Transfer Agent (MTA)

A process which conforms to [SMTP-MTA], which acts as an SMTP server
to accept messages from an MSA or another MTA, and either delivers
them or acts as an SMTP client to relay them to another MTA.

Message User Agent (MUA)

A process which acts (usually on behalf of a user) to compose and
submit new messages, and process delivered messages. In the split-
MUA model, POP or IMAP is used to access delivered messages.

2.2. Conventions Used in this Document

In examples, "C:" is used to indicate lines sent by the client, and
"S:" indicates those sent by the server. Line breaks within a
command example are for editorial purposes only.

Examples use the 'example.net' domain.

The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
in this document are to be interpreted as defined in [KEYWORDS].

3. Message Submission

3.1. Submission Identification

Port 587 is reserved for email message submission as specified in
this document. Messages received on this port are defined to be
submissions. The protocol used is ESMTP [SMTP-MTA, ESMTP], with
additional restrictions as specified here.

While most email clients and servers can be configured to use port
587 instead of 25, there are cases where this is not possible or
convenient. A site MAY choose to use port 25 for message submission,
by designating some hosts to be MSAs and others to be MTAs.

3.2. Message Rejection and Bouncing

MTAs and MSAs MAY implement message rejection rules that rely in part
on whether the message is a submission or a relay.

For example, some sites might configure their MTA to reject all RCPT
TOs for messages that do not reference local users, and configure
their MSA to reject all message submissions that do not come from
authorized users, based on IP address, or authenticated identity.

NOTE: It is better to reject a message than to risk sending one that
is damaged. This is especially true for problems that are
correctable by the MUA, for example, an invalid 'From' field.

If an MSA is not able to determine a return path to the submitting
user, from a valid MAIL FROM, a valid source IP address, or based on
authenticated identity, then the MSA SHOULD immediately reject the
message. A message can be immediately rejected by returning a 550
code to the MAIL FROM command.

Note that a null return path, that is, MAIL FROM:<>, is permitted
and MUST be accepted. (MUAs need to generate null return-path
messages for a variety of reasons, including disposition
notifications.)

Except in the case where the MSA is unable to determine a valid
return path for the message being submitted, text in this
specification which instructs an MSA to issue a rejection code MAY be
complied with by accepting the message and subsequently generating a
bounce message. (That is, if the MSA is going to reject a message for
any reason except being unable to determine a return path, it can
optionally do an immediate rejection or accept the message and then
mail a bounce.)

NOTE: In the normal case of message submission, immediately
rejecting the message is preferred, as it gives the user and MUA
direct feedback. To properly handle delayed bounces the client MUA
must maintain a queue of messages it has submitted, and match bounces
to them.

3.3. Authorized Submission

Numerous methods have been used to ensure that only authorized users
are able to submit messages. These methods include authenticated
SMTP, IP address restrictions, secure IP, and prior POP
authentication.

Authenticated SMTP [SMTP-AUTH] has been proposed. It allows the MSA
to determine an authorization identity for the message submission,
which is not tied to other protocols.

IP address restrictions are very widely implemented, but do not allow
for travellers and similar situations, and can be spoofed.

Secure IP [IPSEC] can also be used, and provides additional benefits
of protection against eavesdropping and traffic analysis.

Requiring a POP [POP3] authentication (from the same IP address)
within some amount of time (for example, 20 minutes) prior to the
start of a message submission session has also been used, but this
does impose restrictions on clients as well as servers which may
cause difficulties. Specifically, the client must do a POP
authentication before an SMTP submission session, and not all clients
are capable and configured for this. Also, the MSA must coordinate
with the POP server, which may be difficult. There is also a window
during which an unauthorized user can submit messages and appear to
be a prior authorized user.

3.4. Enhanced Status Codes

This memo suggests several enhanced status codes [SMTP-CODES] for
submission-specific rejections. The specific codes used are:

5.6.0 Bad content. The content of the header or text is
improper.

5.6.2 Bad domain or address. Invalid or improper domain or address
in MAIL FROM, RCPT TO, or DATA.

5.7.1 Not allowed. The address in MAIL FROM appears to have
insufficient submission rights, or is invalid, or is not
authorized with the authentication used; the address in a
RCPT TO command is inconsistent with the permissions given to
the user; the message data is rejected based on the
submitting user.

5.7.0 Site policy. The message appears to violate site policy in
some way.

4. Mandatory Actions

An MSA MUST do all of the following:

4.1. General Submission Rejection Code

Unless covered by a more precise response code, response code 554 is
to be used to reject a MAIL FROM, RCPT TO, or DATA command that
contains something improper. Enhanced status code 5.6.0 is to be
used if no other code is more specific.

4.2. Ensure All Domains are Fully-Qualified

The MSA MUST ensure that all domains in the envelope are fully-
qualified.

If the MSA examines or alters the message text in way, except to add
trace header fields [SMTP-MTA], it MUST ensure that all domains in
address header fields are fully-qualified.

Reply code 554 is to be used to reject a MAIL FROM, RCPT TO, or DATA
command which contains improper domain references.

NOTE: A frequent local convention is to accept single-level domains
(for example, 'sales') and then to expand the reference by adding the
remaining portion of the domain name (for example, to

'sales.example.net'). Local conventions that permit single-level
domains SHOULD reject, rather than expand, incomplete multi-level
domains, since such expansion is particularly risky.

5. Recommended Actions

The MSA SHOULD do all of the following:

5.1. Enforce Address Syntax

An MSA SHOULD reject messages with illegal syntax in a sender or
recipient envelope address.

If the MSA examines or alters the message text in way, except to add
trace header fields, it SHOULD reject messages with illegal address
syntax in address header fields.

Reply code 501 is to be used to reject a MAIL FROM or RCPT TO command
that contains a detectably improper address.

When addresses are resolved after submission of the message body,
reply code 554 with enhanced status code 5.6.2 is to be used after
end-of-data, if the message contains invalid addresses in the header.

5.2. Log Errors

The MSA SHOULD log message errors, especially apparent
misconfigurations of client software.

Note: It can be very helpful to notify the administrator when
problems are detected with local mail clients. This is another
advantage of distinguishing submission from relay: system
administrators might be interested in local configuration problems,
but not in client problems at other sites.

6. Optional Actions

The MSA MAY do any of the following:

6.1. Enforce Submission Rights

The MSA MAY issue an error response to the MAIL FROM command if the
address in MAIL FROM appears to have insufficient submission rights,
or is not authorized with the authentication used (if the session has
been authenticated).

Reply code 550 with enhanced status code 5.7.1 is used for this
purpose.

6.2. Require Authentication

The MSA MAY issue an error response to the MAIL FROM command if the
session has not been authenticated.

Section 3.3 discusses authentication mechanisms.

Reply code 530 [SMTP-AUTH] is used for this purpose.

6.3. Enforce Permissions

The MSA MAY issue an error response to the RCPT TO command if
inconsistent with the permissions given to the user (if the session
has been authenticated).

Reply code 550 with enhanced status code 5.7.1 is used for this
purpose.

6.4. Check Message Data

The MSA MAY issue an error response to the DATA command or send a
failure result after end-of-data if the submitted message is
syntactically invalid, or seems inconsistent with permissions given
to the user (if known), or violates site policy in some way.

Reply code 554 is used for syntactic problems in the data. Reply
code 501 is used if the command itself is not syntactically valid.
Reply code 550 with enhanced status code 5.7.1 is used to reject
based on the submitting user. Reply code 550 with enhanced status
code 5.7.0 is used if the message violates site policy.

7. Interaction with SMTP Extensions

The following table lists the current standards-track and
Experimental SMTP extensions. Listed are the RFC, name, an
indication as to the use of the extension on the submit port, and a
reference:

RFCName Submission Reference
---- --------------- ---------- ------------------
2197 Pipelining SHOULD [PIPELINING]
2034 Error Codes SHOULD [CODES-EXTENSION]
1985 ETRN MUST NOT [ETRN]
1893 Extended Codes SHOULD [SMTP-CODES]
1891 DSN SHOULD [DSN]
1870 Size MAY [SIZE]
1846 521 MUST NOT [521REPLY]
1845 Checkpoint MAY [Checkpoint]

1830 Binary MAY [CHUNKING]
1652 8-bit MIME SHOULD [8BITMIME]
---- Authentication ------ [SMTP-AUTH]

Future SMTP extensions should explicitly specify if they are valid on
the Submission port.

Some SMTP extensions are especially useful for message submission:

Extended Status Codes [SMTP-CODES], SHOULD be supported and used
according to [CODES-EXTENSION]. This permits the MSA to notify the
client of specific configuration or other problems in more detail
than the response codes listed in this memo. Because some rejections
are related to a site's security policy, care should be used not to
expose more detail than is needed to correct the problem.

[PIPELINING] SHOULD be supported by the MSA.

[SMTP-AUTH] allows the MSA to validate the authority and determine
the identity of the submitting user.

Any references to the DATA command in this memo also refer to any
substitutes for DATA, such as the BDAT command used with [CHUNKING].

8. Message Modifications

Sites MAY modify submissions to ensure compliance with standards and
site policy. This section describes a number of such modifications
that are often considered useful.

NOTE: As a matter of guidance for local decisions to implement
message modification, a paramount rule is to limit such actions to
remedies for specific problems that have clear solutions. This is
especially true with address elements. For example, indiscriminately
appending a domain to an address or element which lacks one typically
results in more broken addresses. An unqualified address must be
verified to be a valid local part in the domain before the domain can
be safely added.

8.1. Add 'Sender'

The MSA MAY add or replace the 'Sender' field, if the identity of the
sender is known and this is not given in the 'From' field.

The MSA MUST ensure that any address it places in a 'Sender' field is
in fact a valid mail address.

8.2. Add 'Date'

The MSA MAY add a 'Date' field to the submitted message, if it lacks
it, or correct the 'Date' field if it does not conform to [MESSAGE-
FORMAT] syntax.

8.3. Add 'Message-ID'

The MSA MAY add or replace the 'Message-ID' field, if it lacks it, or
it is not valid syntax (as defined by [MESSAGE-FORMAT]).

8.4. Transfer Encode

The MSA MAY apply transfer encoding to the message according to MIME
conventions, if needed and not harmful to the MIME type.

8.5. Sign the Message

The MSA MAY (digitally) sign or otherwise add authentication
information to the message.

8.6. Encrypt the Message

The MSA MAY encrypt the message for transport to reflect
organizational policies.

NOTE: To be useful, the addition of a signature and/or encryption by
the MSA generally implies that the connection between the MUA and MSA
must itself be secured in some other way, e.g., by operating inside
of a secure environment, by securing the submission connection at the
transport layer, or by using an [SMTP-AUTH] mechanism that provides
for session integrity.

8.7. Resolve Aliases

The MSA MAY resolve aliases (CNAME records) for domain names, in the
envelope and optionally in address fields of the header, subject to
local policy.

NOTE: Unconditionally resolving aliases could be harmful. For
example, if www.example.net and ftp.example.net are both aliases for
mail.example.net, rewriting them could lose useful information.

8.8. Header Rewriting

The MSA MAY rewrite local parts and/or domains, in the envelope and
optionally in address fields of the header, according to local
policy. For example, a site may prefer to rewrite 'JRU' as '

J.Random.User' in order to hide logon names, and/or to rewrite '
squeeky.sales.example.net' as 'zyx.example.net' to hide machine names
and make it easier to move users.

However, only addresses, local-parts, or domains which match specific
local MSA configuration settings should be altered. It would be very
dangerous for the MSA to apply data-independent rewriting rules, such
as always deleting the first element of a domain name. So, for
example, a rule which strips the left-most element of the domain if
the complete domain matches '*.foo.example.net' would be acceptable.

9. Security Considerations

Separation of submission and relay of messages can allow a site to
implement different policies for the two types of services, including
requiring use of additional security mechanisms for one or both. It
can do this in a way which is simpler, both technically and
administratively. This increases the likelihood that policies will
be applied correctly.

Separation also can aid in tracking and preventing unsolicited bulk
email.

For example, a site could configure its MSA to require authentication
before accepting a message, and could configure its MTA to reject all
RCPT TOs for non-local users. This can be an important element in a
site's total email security policy.

If a site fails to require any form of authorization for message
submissions (see section 3.3 for discussion), it is allowing open use
of its resources and name; unsolicited bulk email can be injected
using its facilities.

10. Acknowledgments

This updated memo has been revised in part based on comments and
discussions which took place on and off the IETF-Submit mailing list.
The help of those who took the time to review the draft and make
suggestions is appreciated, especially that of Dave Crocker, Ned
Freed, Keith Moore, John Myers, and Chris Newman.

Special thanks to Harald Alvestrand, who got this effort started.

11. References

[521REPLY] Durand, A. and F. Dupont, "SMTP 521 Reply Code",
RFC1846, September 1995.

[8BITMIME] Klensin, J., Freed, N., Rose, M., Stefferud, E. and
D. Crocker, "SMTP Service Extension for 8bit-
MIMEtransport", RFC1652, July 1994.

[ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", RFC2234, November
1997.

[CHECKPOINT] Crocker, D., Freed, N. and A. Cargille, "SMTP
Service Extension for Checkpoint/Restart", RFC
1845, September 1995.

[CHUNKING] Vaudreuil, G., "SMTP Service Extensions for
Transmission of Large and Binary MIME Messages",
RFC1830, August 1995.

[CODES-EXTENSION] Freed, N., "SMTP Service Extension for Returning
Enhanced Error Codes", RFC2034, October 1996.

[DSN] Moore, K., "SMTP Service Extension for Delivery
Status Notifications", RFC1891, January 1996.

[ESMTP] Klensin, J., Freed, N., Rose, M., Stefferud, E. and
D. Crocker, "SMTP Service Extensions", STD 10, RFC
1869, November 1995.

[ETRN] De Winter, J., "SMTP Service Extension for Remote
Message Queue Starting", RFC1985, August 1996.

[HEADERS] Palme, J., "Common Internet Message Headers", RFC
2076, February 1997.

[IPSEC] Atkinson, R., "Security Architecture for the
Internet Protocol", RFC1825, August 1995.

[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.

[MESSAGE-FORMAT] Crocker, D., "Standard for the format of ARPA
Internet text messages", STD 11, RFC822, August
1982;

Braden, R., Editor, "Requirements for Internet
Hosts -- Application and Support", STD 3, RFC1123,
October 1989.

[PIPELINING] Freed, N., "SMTP Service Extension for Command
Pipelining", RFC2197, September 1997.

[POP3] Myers, J. and M. Rose, "Post Office Protocol --
Version 3", STD 53, RFC1939, May 1996.

[SIZE] Klensin, J., Freed, N. and K. Moore, "SMTP Service
Extension for Message Size Declaration", STD 10,
RFC1870, November 1995.

[SMTP-AUTH] Myers, J., "SMTP Service Extension for
Authentication", Work in Progress.

[SMTP-CODES] Vaudreuil, G., "Enhanced Mail System Status Codes",
RFC1893, January 1996.

[SMTP-MTA] Postel, J., "Simple Mail Transfer Protocol", STD
10, RFC821, August 1982.

Partridge, C., "Mail Routing and the Domain
System", STD 14, RFC974, January 1986.

Braden, R., Editor, "Requirements for Internet
Hosts -- Application and Support", STD 3, RFC1123,
October 1989.

12. Authors' Addresses

Randall Gellens
QUALCOMM Incorporated
6455 Lusk Blvd.
San Diego, CA 92121-2779
U.S.A.

Phone: +1 619 651 5115
Fax: +1 619 651 5334
EMail: Randy@Qualcomm.Com

John C. Klensin
MCI Telecommunications
800 Boylston St, 7th floor
Boston, MA 02199
USA

Phone: +1 617 960 1011
EMail: klensin@mci.net

13. Full Copyright Statement

Copyright (C) The Internet Society (1998). All Rights Reserved.

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