This is a purely informative rendering of an RFC that includes verified errata. This rendering may not be used as a reference.
The following 'Verified' errata have been incorporated in this document:
EID 1078
Network Working Group R. Gellens
Request for Comments: 4409 QUALCOMM
Obsoletes: 2476 J. Klensin
Category: Standards Track April 2006
Message Submission for Mail
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 (2006).
Abstract
This memo splits message submission from message relay, allowing each
service to operate according to its own rules (for security, policy,
etc.), and specifies what actions are to be taken by a submission
server.
Message relay and final delivery are unaffected, and continue to use
SMTP over port 25.
When conforming to this document, message submission uses the
protocol specified here, normally over port 587.
This separation of function offers a number of benefits, including
the ability to apply specific security or policy requirements.
Table of Contents
1. Introduction ....................................................3
2. Document Information ............................................4
2.1. Definitions of Terms Used in This Memo .....................4
2.2. Conventions Used in This Document ..........................5
3. Message Submission ..............................................5
3.1. Submission Identification ..................................5
3.2. Message Rejection and Bouncing .............................5
3.3. Authorized Submission ......................................6
4. Mandatory Actions ...............................................7
4.1. General Submission Rejection Code ..........................7
4.2. Ensure All Domains Are Fully-Qualified .....................7
4.3. Require Authentication .....................................8
5. Recommended Actions .............................................8
5.1. Enforce Address Syntax .....................................8
5.2. Log Errors .................................................8
6. Optional Actions ................................................9
6.1. Enforce Submission Rights ..................................9
6.2. Enforce Permissions ........................................9
6.3. Check Message Data .........................................9
6.4. Support for the Postmaster Address .........................9
7. Interaction with SMTP Extensions ...............................10
8. Message Modifications ..........................................11
8.1. Add 'Sender' ..............................................11
8.2. Add 'Date' ................................................11
8.3. Add 'Message-ID' ..........................................11
8.4. Transfer Encode ...........................................11
8.5. Sign the Message ..........................................11
8.6. Encrypt the Message .......................................12
8.7. Resolve Aliases ...........................................12
8.8. Header Rewriting ..........................................12
9. Security Considerations ........................................12
10. IANA Considerations ...........................................13
11. Acknowledgements ..............................................13
12. Normative References ..........................................14
13. Informative References ........................................14
1. Introduction
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
that accepts message submissions from MUAs is termed a Message
Submission Agent (MSA).
In order to permit unconstrained communications, SMTP is not often
authenticated during message relay.
Authentication and authorization of initial submissions have become
increasingly important, driven by changes in security requirements
and rising expectations that submission servers take responsibility
for the message traffic they originate.
For example, due to the prevalence of machines that have worms,
viruses, or other malicious software that generate large amounts of
spam, many sites now prohibit outbound traffic on the standard SMTP
port (port 25), funneling all mail submissions through submission
servers.
In addition to authentication and authorization issues, messages
being submitted are in some cases finished (complete) messages, and
in other cases are unfinished (incomplete) in one or more aspects.
Unfinished messages may 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 (e.g., 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, e.g., to
conceal local name or address spaces. 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 do the following:
* 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.
2. Document Information
2.1. Definitions of Terms Used in This Memo
Many of the concepts and terms used in this document are defined in
[SMTP-MTA]; familiarity with those documents is assumed here.
Fully-Qualified
Containing or consisting of a domain that can be globally resolved
using the Domain Name Service; that is, not a local alias or partial
specification.
Message Submission Agent (MSA)
A process that conforms to this specification. An MSA 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 that conforms to [SMTP-MTA]. An MTA 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 that acts (often on behalf of a user and with a user
interface) to compose and submit new messages, and process delivered
messages.
For delivered messages, the receiving MUA may obtain and process the
message according to local conventions or, in what is commonly
referred to as a split-MUA model, Post Office Protocol [POP3] or IMAP
[IMAP4] is used to access delivered messages, whereas the protocol
defined here (or SMTP) is used to submit 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 or allowances as specified here.
Although 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 MTAs to reject all RCPT
commands for messages that do not reference local users, and
configure their MSA to reject all message submissions that do not
come from authorized users, with authorization based either on
authenticated identity or the submitting endpoint being within a
protected IP environment.
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 command.
Note that a null return path, that is, MAIL FROM:<>, is permitted and
MUST NOT in itself be cause for rejecting a message. (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 that 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
needs to maintain a queue of messages it has submitted, and match
bounces to them. Note that many contemporary MUAs do not have this
capability.
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 other tunnels, and prior
POP authentication.
Authenticated SMTP [SMTP-AUTH] has seen widespread deployment. It
allows the MSA to determine an authorization identity for the message
submission, one that is not tied to other protocols.
IP address restrictions are very widely implemented, but do not allow
for travelers and similar situations, and can be easily spoofed
unless all transport paths between the MUA and MSA are trustworthy.
Secure IP [IPSEC], and other encrypted and authenticated tunneling
techniques, can also be used and provide additional benefits of
protection against eavesdropping and traffic analysis.
Requiring a POP [POP3] authentication (from the same IP address)
within some amount of time (e.g., 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
previously authorized user. Since it is dependent on the MUA's IP
addresses, this technique is substantially as subject to IP address
spoofing as validation based on known IP addresses alone (see above).
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, RCPT, or DATA command that contains
something improper.
4.2. Ensure All Domains Are Fully-Qualified
The MSA MUST ensure that all domains in the SMTP envelope are fully-
qualified.
If the MSA examines or alters the message text in any 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, RCPT, or DATA command
that contains improper domain references.
A frequent local convention is to accept single-level domains (e.g.,
'sales') and then to expand the reference by adding the remaining
portion of the domain name (e.g., to 'sales.example.net'). Local
conventions that permit single-level domains SHOULD reject, rather
than expand, incomplete multi-level domains (e.g., 'squeaky.sales'),
since such expansion is particularly risky.
4.3. Require Authentication
The MSA MUST by default issue an error response to the MAIL command
if the session has not been authenticated using [SMTP-AUTH], unless
it has already independently established authentication or
authorization (such as being within a protected subnetwork).
Section 3.3 discusses authentication mechanisms.
Reply code 530 [SMTP-AUTH] is used for this purpose.
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 SMTP 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 or RCPT command that
contains a detectably improper address.
When addresses are resolved after submission of the message body,
reply code 554 (with a suitable enhanced status code from
[SMTP-CODES]) is 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.
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.
Note that it is important to impose limits on such logging to prevent
certain forms of denial of service (DoS) attacks.
6. Optional Actions
The MSA MAY do any of the following:
6.1. Enforce Submission Rights
The MSA MAY issue an error response to a MAIL 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 an appropriate enhanced status code per
[SMTP-CODES], such as 5.7.1, is used for this purpose.
6.2. Enforce Permissions
The MSA MAY issue an error response to a RCPT command if inconsistent
with the permissions given to the user (if the session has been
authenticated).
Reply code 550 with an appropriate enhanced status code per
[SMTP-CODES], such as 5.7.1, is used for this purpose.
6.3. 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 an appropriate enhanced status code per
[SMTP-CODES] (such as 5.7.1) is used to reject based on the
submitting user. Reply code 550 with an appropriate enhanced status
code (such as 5.7.0) is used if the message violates site policy.
6.4. Support for the Postmaster Address
If appropriate under local conditions and to facilitate conformance
with the "postmaster" requirements of [SMTP-MTA], the MSA MAY permit
a reduced degree of authentication for mail addressed to the
"postmaster" (or one of its alternate spelling forms, see
[SMTP-MTA]), in one or more domains, as compared to requirements
enforced for other addresses. Among other benefits, this provides an
address of last resort that can be used by authorized users to report
problems that otherwise prevent them from submitting mail.
7. Interaction with SMTP Extensions
The following table lists the current standards-track and
Experimental SMTP extensions. Listed are the EHLO keyword, name, an
indication as to the use of the extension on the submit port, and a
reference:
Keyword Name Submission Reference
---------- -------------------------- ---------- ----------------
PIPELINING Pipelining SHOULD [PIPELINING]
ENHANCEDSTATUSCODES Enhanced Status Codes SHOULD [CODES-EXTENSION]
ETRN Extended Turn MUST NOT [ETRN]
... Extended Codes SHOULD [SMTP-CODES]
DSN Delivery Status Notification SHOULD [DSN]
SIZE Message size MAY [SIZE]
... 521 reply code MUST NOT [521REPLY]
CHECKPOINT Checkpoint/Restart MAY [CHECKPOINT]
BINARYMIME Binary MIME MAY [CHUNKING]
CHUNKING Chunking MAY [CHUNKING]
8BITMIME Use 8-bit data SHOULD [8BITMIME]
AUTH Authentication MUST [SMTP-AUTH]
STARTTLS Start TLS MAY [Start-TLS]
NO-SOLICITING Notification of no soliciting MAY [No-soliciting]
And a new reference in Section 13:
[No-soliciting] C. Malamud, "A No Soliciting Simple Mail Transfer
Protocol (SMTP) Service Extension", RFC 3865,
September 2004.
EID 1078 (Verified) is as follows:Section: 7
Original Text:
NO-SOLICITING Notification of no soliciting MAY [Msg-Track]
Corrected Text:
NO-SOLICITING Notification of no soliciting MAY [No-soliciting]
And a new reference in Section 13:
[No-soliciting] C. Malamud, "A No Soliciting Simple Mail Transfer
Protocol (SMTP) Service Extension", RFC 3865,
September 2004.
Notes:
Section 13 says:
[Msg-Track] Allman, E. and T. Hansen, "SMTP Service Extension for Message Tracking", RFC 3885, September 2004.
Which is not correct for NO-SOLICITING, which is defined in RFC 3865
MTRK Message Tracking MAY [Msg-Track]
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 to unauthenticated senders than is needed
[PIPELINING] SHOULD be supported by the MSA.
[SMTP-AUTH] allows the MSA to validate the authority and determine
the identity of the submitting user and MUST be supported by the MSA.
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 that 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.
Any message forwarded or delivered by the MSA MUST conform to the
requirements of [SMTP-MTA] and [MESSAGE-FORMAT].
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 SHOULD add or replace the 'Message-ID' field, if it lacks it,
or it is not valid syntax (as defined by [MESSAGE-FORMAT]). Note
that a number of clients still do not generate Message-ID fields.
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, for example, 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
SMTP 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 SMTP 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 login names, and/or to rewrite
'squeaky.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 that strips the left-most element of the domain, if
the complete domain matches '*.foo.example.net', would be acceptable.
The MSA MUST NOT rewrite a forward-pointing (destination) address in
a way that violates the constraints of [SMTP-MTA] on modifications of
local-parts.
9. Security Considerations
Separation of submission and relay of messages allows 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 mail servers such that the
MSA requires authentication before accepting a message, and the MTA
rejects all RCPT commands 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.
Section 3 includes further discussion of issues with some
authentication methods.
Section 5.2 includes a cautionary note that unlimited logging can
enable certain forms of denial of service attacks.
10. IANA Considerations
The registration for port 587 has been updated to refer to this memo
rather than RFC 2476.
11. Acknowledgements
Nathaniel Borenstein and Barry Leiba were instrumental in the
development of this update to RFC 2476.
The original memo (RFC 2476) was developed 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 that document
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.
12. Normative References
[ESMTP] Klensin, J., Freed, N., Rose, M., Stefferud, E.,
and D. Crocker, "SMTP Service Extensions", STD 10,
RFC 1869, November 1995.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[SMTP-MTA] Postel, J., "Simple Mail Transfer Protocol", STD
10, RFC 821, August 1982.
Partridge, C., "Mail routing and the domain
system", STD 10, RFC 974, January 1986.
Braden, R., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123, October
1989.
Klensin, J., "Simple Mail Transfer Protocol", RFC
2821, April 2001.
13. Informative References
[521REPLY] Durand, A. and F. Dupont, "SMTP 521 Reply Code",
RFC 1846, September 1995.
[8BITMIME] Klensin, J., Freed, N., Rose, M., Stefferud, E.,
and D. Crocker, "SMTP Service Extension for 8bit-
MIMEtransport", RFC 1652, July 1994.
[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",
RFC 3030, December 2000.
[CODES-EXTENSION] Freed, N., "SMTP Service Extension for Returning
Enhanced Error Codes", RFC 2034, October 1996.
[DSN] Moore, K., "Simple Mail Transfer Protocol (SMTP)
Service Extension for Delivery Status Notifications
(DSNs)", RFC 3461, January 2003.
[ETRN] De Winter, J., "SMTP Service Extension for Remote
Message Queue Starting", RFC 1985, August 1996.
[IMAP4] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
VERSION 4rev1", RFC 3501, March 2003.
[IPSEC] Kent, S. and R. Atkinson, "Security Architecture
for the Internet Protocol", RFC 2401, November
1998.
[MESSAGE-FORMAT] Crocker, D., "Standard for the format of ARPA
Internet text messages", STD 11, RFC 822, August
1982.
Braden, R., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123, October
1989.
Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
[Msg-Track] Allman, E. and T. Hansen, "SMTP Service Extension
for Message Tracking", RFC 3885, September 2004.
[PIPELINING] Freed, N., "SMTP Service Extension for Command
Pipelining", STD 60, RFC 2920, September 2000.
[POP3] Myers, J. and M. Rose, "Post Office Protocol -
Version 3", STD 53, RFC 1939, May 1996.
[SIZE] Klensin, J., Freed, N., and K. Moore, "SMTP Service
Extension for Message Size Declaration", STD 10,
RFC 1870, November 1995.
[SMTP-AUTH] Myers, J., "SMTP Service Extension for
Authentication", RFC 2554, March 1999.
[SMTP-CODES] Vaudreuil, G., "Enhanced Mail System Status Codes",
RFC 3463, January 2003.
[Start-TLS] Hoffman, P., "SMTP Service Extension for Secure
SMTP over Transport Layer Security", RFC 3207,
February 2002.
Authors' Addresses
Randall Gellens
QUALCOMM Incorporated
5775 Morehouse Drive
San Diego, CA 92121-2779
USA
EMail: rg+ietf@qualcomm.com
John C. Klensin
1770 Massachusetts Ave, #322
Cambridge, MA 02140
USA
EMail: john+ietf@jck.com
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