RFC 3848

RFC 3848: ESMTP and LMTP Transmission Types Registration 💖

Network Working Group 🌸 Request for Comments: 3848 🎀 Author: Chris Newman, Sun Microsystems 👑 Category: Standards Track ✨ Date: July 2004 📅

Okay so the title block doesn't list anything it obsoletes, and it doesn't list anything it updates either. BUT it does update a registry, which we'll get to in Section 1 🦋

Status of this Memo ✨

Okay, important bit. This doc specifies an Internet standards track protocol for the Internet community, and it's asking everyone for discussion and suggestions for improvements 🫶 For the current standardization state and status of this protocol, check the latest edition of the "Internet Official Protocol Standards" (STD 1). Distribution of this memo is unlimited, so share it with your whole group chat 💅

Copyright Notice 💜

Copyright (C) The Internet Society (2004). 🌈

Abstract 🦋

This registers seven new mail transmission types (ESMTPA, ESMTPS, ESMTPSA, LMTP, LMTPA, LMTPS, LMTPSA) for use in the "with" clause of a Received header in an Internet message. So basically, these seven new names are going into the trace info on your emails 📬✨

1. IANA Considerations 🌸

Okay so, as SMTP [2] directs, IANA keeps a registry [7] of "WITH protocol types" for the "with" clause of the Received header in an Internet message. Right now that registry has SMTP [6] and ESMTP [2] in it. This specification updates the registry like this:

• The new keyword "ESMTPA" means ESMTP is being used when the SMTP AUTH [3] extension is also used and authentication is successfully achieved 🔐
• The new keyword "ESMTPS" means ESMTP is being used when STARTTLS [1] is also successfully negotiated to give a strong transport encryption layer 🔒
• The new keyword "ESMTPSA" means ESMTP is being used when both STARTTLS and SMTP AUTH are successfully negotiated (the combination of ESMTPS and ESMTPA) 👑
• The new keyword "LMTP" means LMTP [4] is being used 🧚
• The new keyword "LMTPA" means LMTP is being used when the SMTP AUTH extension is also used and authentication is successfully achieved 🔐
• The new keyword "LMTPS" means LMTP is being used when STARTTLS is also successfully negotiated to give a strong transport encryption layer 🔒
• The new keyword "LMTPSA" means LMTP is being used when both STARTTLS and SMTP AUTH are successfully negotiated (the combination of LSMTPS and LSMTPA) 💀
• The references for the ESMTP and SMTP entries in the registry should be updated to the latest specification [2], since both RFC 821 and RFC 1869 [5] are obsoleted by RFC 2821 📚

2. Implementation Experience 🍓

The ESMTPA, ESMTPS and ESMTPSA keywords have been implemented in deployed email server software for several years, and no problems have been reported with their use. Iconic, honestly 🔥

3. Security Considerations 💀

Okay, this section is serious, so grab a snack and pay attention 🙈

Using these extra keywords gives trace information showing when various high-level security framing protocols are used for hop-to-hop transport of email, without exposing the specifics of the security mechanism. This trace info is an informal way to track how these mechanisms are being deployed across the Internet, and it can help with after-the-fact diagnosis of email abuse 🕵️‍♀️

But here's the tea. These keywords are not normally protected in transport, which means an active attacker can modify them. They also don't tell you the specifics of the mechanism used, so they don't give any real-world security assurance. They should not be used for mail filtering or relaying decisions except in very controlled environments. They're cryptic and hidden in trace headers that are mostly used to diagnose email problems, so it's not expected that they'll mislead end users into a false sense of security 🙅‍♀️ If you want information you can actually rely on, correlate the Received headers with the logs of the various Mail Transfer Agents the message passed through 📋

The trace info from these keywords, plus the rest of the Received header, is a huge benefit when doing after-the-fact diagnosis of email abuse or problems. Unfortunately, some people try to hide info about their internal servers by stripping useful information out of Received headers. That makes it harder to fix security abuses after they happen, and the result is usually less overall security for everyone. Not it, bestie 😭

4. References 📚

4.1. Normative References 💜

[1] Hoffman, P., "SMTP Service Extension for Secure SMTP over Transport Layer Security", RFC 3207, February 2002.

[2] Klensin, J., Ed., "Simple Mail Transfer Protocol", RFC 2821, April 2001.

[3] Myers, J., "SMTP Service Extension for Authentication", RFC 2554, March 1999.

[4] Myers, J., "Local Mail Transfer Protocol", RFC 2033, October 1996.

4.2. Informative References 🦋

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

[6] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821, August 1982.

4.3. URIs 🌈

[7] <http://www.iana.org/assignments/mail-parameters>

Author's Address 👑

Chris Newman Sun Microsystems 1050 Lakes Drive West Covina, CA 91790 US

EMail: chris.newman@sun.com 💌

Full Copyright Statement 🎀

Copyright (C) The Internet Society (2004). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights.

This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 🫶

Intellectual Property 💅

The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document, or the extent to which any license under such rights might or might not be available. It also doesn't represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat, any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. 🧚

Acknowledgement 🌸

Funding for the RFC Editor function is currently provided by the Internet Society. ✨