RFC 4409
Message Submission for Mail
RFC 4409: Message Submission for Mail 💌
Network Working Group 🎀 This one's from the Network Working Group, written by R. Gellens (QUALCOMM) and J. Klensin 🧚
It's on the Standards Track 👑 and dates from April 2006 🍓 It obsoletes RFC 2476, so 2476 is out and this is the new version 💅
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 🌸
Okay so this memo splits message submission away from message relay. That means each service gets to run by its own rules for security, policy, and so on 🔥 It also spells out what a submission server has to do.
Message relay and final delivery stay exactly as they are, and they keep using SMTP over port 25 🙌
When you're following this document, message submission uses the protocol described here, normally over port 587 💜
Splitting things up like this has a bunch of perks, including being able 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 ........................................141. Introduction 🧚
Okay bestie, so SMTP was originally built as a message transfer protocol. Its whole job was routing (when needed) and delivering finished, complete messages 📬
Message Transfer Agents (MTAs) aren't really supposed to mess with the message text. The exception is adding 'Received', 'Return-Path', and other header fields that [SMTP-MTA] says they need to add 💅
But SMTP is also super popular now as a message submission protocol 🔥 That means it's how Message User Agents (MUAs) get brand-new messages into the MTA routing network. The part that takes in those submissions from MUAs is called a Message Submission Agent (MSA) 💖
To keep communication wide open, SMTP usually isn't authenticated during message relay 🦋
But authentication and authorization for initial submissions have become way more important 😭 Security needs changed, and people now expect submission servers to take responsibility for the traffic they start 🫶
Here's a big example. A ton of machines are infected with worms, viruses, or other malware that blast out huge amounts of spam 💀 Because of that, lots of sites now block outbound traffic on the standard SMTP port (port 25) and push all mail submissions through submission servers instead 🙈
Authentication and authorization aren't the only issues, either. Sometimes the messages being submitted are already finished, and sometimes they're unfinished in one or more ways 🌈 Unfinished messages might need completing so they follow [MESSAGE-FORMAT] and later requirements. For instance, a message might be missing a proper 'Date' header field, and domains might not be fully qualified ✨ Sometimes the MUA just can't make a finished message at all, like when it doesn't know its time zone 🍓
Even when submitted messages are complete, local site policy might want the text examined or changed somehow. An example would be hiding local name or address spaces 👀 Those kinds of completions or changes have been shown to cause harm when downstream MTAs do them (MTAs after the first-hop submission MTA), and they're generally considered outside what standardized MTA functionality should cover 🙅♀️
Splitting messages into submissions and transfers makes life easier for developers and network admins in these ways:
- 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, which keeps each code base more straightforward and allows 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 🚀
So this memo describes a low-cost, deterministic way for messages to be identified as submissions, and it spells out what a submission server has to do with them 👑
2. Document Information 🎀
2.1. Definitions of Terms Used in This Memo 📖
Lots of the concepts and terms in this doc are already defined in [SMTP-MTA], so 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. In other words, it's not a local alias or a 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 then 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 then 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 to process delivered messages 💅
For delivered messages, the receiving MUA may get and process the message according to local conventions. Or, in what's commonly called a split-MUA model, Post Office Protocol [POP3] or IMAP [IMAP4] is used to access delivered messages, while the protocol defined here (or SMTP) is used to submit messages 🧠
2.2. Conventions Used in This Document 🌷
In examples, "C:" marks lines sent by the client, and "S:" marks lines sent by the server. Line breaks inside a command example are just for editorial purposes 🫧
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 is ESMTP [SMTP-MTA, ESMTP], with additional restrictions or allowances as specified here 🔐
Almost all email clients and servers can be set up to use port 587 instead of 25. But there are cases where that isn't possible or convenient. A site MAY choose to use port 25 for message submission by designating some hosts as MSAs and others as MTAs 🧩
3.2. Message Rejection and Bouncing 🚫
MTAs and MSAs MAY implement message rejection rules that depend partly on whether the message is a submission or a relay 🛑
For example, some sites might set their MTAs to reject all RCPT commands for messages that don't reference local users. They might also set their MSA to reject all message submissions that don't come from authorized users. Authorization could be based on authenticated identity, or on the submitting endpoint being inside 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 the MUA can fix itself, like an invalid 'From' field. 🙅♀️
If an MSA can't figure out a return path to the submitting user, whether from a valid MAIL FROM, a valid source IP address, or authenticated identity, then the MSA SHOULD immediately reject the message 😤 To do that, it returns a 550 code to the MAIL command 💀
Now, a null return path, which is MAIL FROM:<>, is allowed. It MUST NOT by itself be a reason to reject a message. (MUAs need to generate null return-path messages for a bunch of reasons, including disposition notifications 📨)
Except when the MSA can't determine a valid return path for the message being submitted, any text in this specification that tells an MSA to issue a rejection code MAY be followed by accepting the message and then generating a bounce message instead 🪃 In other words, if the MSA is going to reject a message for any reason besides not being able to find a return path, it can either reject it right away or accept it and send a bounce later 🎀
NOTE: In the normal case of message submission, rejecting immediately is preferred, because it gives the user and MUA direct feedback 💅 To handle delayed bounces properly, the client MUA needs to keep a queue of the messages it has submitted and match bounces back to them. Heads up, though: lots of contemporary MUAs can't do this yet 😭
3.3. Authorized Submission 🔐
okay so there are a bunch of ways to make sure only the right people can submit messages, and it's giving security checkpoint 💅. The main ones are authenticated SMTP, IP address restrictions, secure IP and other tunnels, and prior POP authentication 🎀.
Authenticated SMTP [SMTP-AUTH] is literally everywhere now ✨. It lets the MSA figure out an authorization identity for the submission, and that identity isn't tied to any other protocol. Iconic 👑.
IP address restrictions are super common, but they're not very travel-friendly 🧳, so anyone who's on the move or in a similar situation gets left out 😭. They're also easy to spoof, unless every transport path between the MUA and MSA is trustworthy 🙈.
Secure IP [IPSEC], plus other encrypted and authenticated tunneling techniques, can also be used 🔒🦋. They come with the bonus of protecting against eavesdropping and traffic analysis, so that's a whole vibe 💜.
Requiring a POP [POP3] authentication from the same IP address within some amount of time (e.g., 20 minutes) before a message submission session starts has also been tried 🧚. Here's the tea though: it puts restrictions on clients and servers, which can cause real difficulties 😬. The client has to do a POP authentication before every SMTP submission session, and not all clients can do that or are set up for it. The MSA also has to coordinate with the POP server, which can be tricky 🫠. There's also a window where an unauthorized user can submit messages and look like a previously authorized user 👀. Since this depends on the MUA's IP addresses, it's basically as vulnerable to IP address spoofing as checking known IP addresses alone (see above) 💀.
4. Mandatory Actions 🚨
An MSA MUST do all of the following:
4.1. General Submission Rejection Code
Unless a more precise response code covers it, response code 554 is 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 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 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 that expansion is particularly risky 😭🚩.
4.3. Require Authentication 🔑
The MSA MUST by default issue an error response to the MAIL command if the session hasn't been authenticated using [SMTP-AUTH], unless it has already independently established authentication or authorization (such as being within a protected subnetwork) 🏰.
Section 3.3 talks about 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 used to reject a MAIL or RCPT command that contains a detectably improper address 🚫.
When addresses are resolved after the message body is submitted, 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 super helpful to notify the administrator when problems are detected with local mail clients 🧑💻. This is another perk of separating submission from relay: sysadmins might care about local configuration problems, but not about client problems at other sites 🌍.
Note that it's important to put limits on this logging so certain denial of service (DoS) attacks can't happen 🛡️😤.
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 isn't 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 it's 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, seems inconsistent with the 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 isn't 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 it's appropriate under local conditions, and it helps with conformance to the "postmaster" requirements of [SMTP-MTA], the MSA MAY allow a reduced degree of authentication for mail addressed to "postmaster" (or one of its alternate spelling forms, see [SMTP-MTA]) in one or more domains, compared to the requirements enforced for other addresses 💌. Among other benefits, this gives you a last-resort address that authorized users can use to report problems that would otherwise stop them from submitting mail 🆘🫶.
7. Interaction with SMTP Extensions 🧩
The following table lists the current standards-track and Experimental SMTP extensions 📋. It shows the EHLO keyword, the name, whether the extension is used 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 [Msg-Track]
MTRK Message Tracking MAY [Msg-Track]Future SMTP extensions SHOULD explicitly specify whether they're 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 lets the MSA tell the client about 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] lets the MSA validate the authority and determine the identity of the submitting user, and it 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 make sure they comply with standards and site policy 💅. This section describes a number of modifications that are often considered useful 🌈.
NOTE: As guidance for local decisions about message modification, the big rule is to limit these actions to fixes for specific problems that have clear solutions 🚨. This is especially true for address elements. For example, blindly appending a domain to an address or element that lacks one usually makes more broken addresses 💀. An unqualified address must be verified as 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 it isn't given in the 'From' field 🙋♀️.
The MSA MUST ensure that any address it places in a 'Sender' field is actually a valid mail address 📧.
8.2. Add 'Date' 📅
The MSA MAY add a 'Date' field to the submitted message if it lacks one, or correct the 'Date' field if it doesn't conform to [MESSAGE-FORMAT] syntax ⏰.
8.3. Add 'Message-ID' 🆔
The MSA SHOULD add or replace the 'Message-ID' field if it lacks one, or if it isn't valid syntax (as defined by [MESSAGE-FORMAT]) 🪄. Note that a number of clients still don't generate Message-ID fields 😤.
8.4. Transfer Encode 🔄
The MSA MAY apply transfer encoding to the message according to MIME conventions, if it's needed and not harmful to the MIME type 🧃.
8.5. Sign the Message ✍️
The MSA MAY (digitally) sign the message or otherwise add authentication information to it 🔏✨.
8.6. Encrypt the Message 🕵️♀️
The MSA MAY encrypt the message for transport to reflect organizational policies 🔐💜.
NOTE: For this to be useful, adding a signature and/or encryption at the MSA generally means the connection between the MUA and MSA must itself be secured in some other way 🛡️. For example, it could be operating inside a secure environment, securing the submission connection at the transport layer, or using an [SMTP-AUTH] mechanism that provides 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 ✨
okay so the MSA MAY rewrite local parts and/or domains in the SMTP envelope, and optionally in the address fields of the header, based on local policy 💅 for example, a site might wanna rewrite 'JRU' as 'J.Random.User' so login names stay hidden 🙈 and/or rewrite 'squeaky.sales.example.net' as 'zyx.example.net' to hide machine names and make it easier to move users around 🦋
BUT ngl, only addresses, local-parts, or domains that match specific local MSA configuration settings should be altered. it would be sooo dangerous for the MSA to apply data-independent rewriting rules, like 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. no cap, this one is a hard no 🚫
9. Security Considerations 🔐
separating submission and relay of messages lets a site run different policies for each type of service, including requiring extra security mechanisms for one or both 🧚 and it can do this in a way that's simpler, both technically and administratively 💖 that means policies are more likely to actually get applied correctly ✨
separation also helps with tracking and preventing unsolicited bulk email 📬
for example, a site could set up its mail servers so the MSA requires authentication before accepting a message, and the MTA rejects all RCPT commands for non-local users 🎀 this can be a super important piece of 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's basically allowing open use of its resources and name 😭 and unsolicited bulk email can get injected using its facilities 💀
section 3 has more discussion on 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 developing this update to RFC 2476 💜
the original memo (RFC 2476) was developed partly based on comments and discussions that happened on and off the IETF-Submit mailing list 📨 the help of everyone who reviewed that document and made suggestions is appreciated, especially Dave Crocker, Ned Freed, Keith Moore, John Myers, and Chris Newman 🌈
special thanks to Harald Alvestrand, who got this whole 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.comFull Copyright Statement ©
Copyright (C) The Internet Society (2006).
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