Dec 7: Revision of the Core Email Specifications

This is the seventh post in the Fastmail Advent 2024 series. The previous post was Dec 6: Twoskip and more. The next post is Dec 8: Guiding principles.

There are many specifications that form the current internet messaging ecosystem, but the most integral of these are RFC 5321 and RFC 5322. RFC 5321 specifies the protocol for the transport of electronic mail messages on the internet, known as the Simple Mail Transfer Protocol (SMTP). RFC 5322 specifies the base syntax of these messages, known as the Internet Message Format (IMF). These documents were both published in 2008, and each are the third version of the “core” specifications (see RFCs 821 / 2821 and RFCs 822 / 2822 respectively).

Over the sixteen years since publication, several errata have been filed against both documents. As a result, an effort is currently underway within the Internet Engineering Task Force (IETF) EMAILCORE working group to revise these specifications yet again. Per its charter, the working group is tasked with updating the documents with “corrections and clarifications only, with a strong emphasis on keeping these minimal and avoiding broader changes to terminology or document organization”.

Work on the revision to the transport protocol specification (RFC 5321bis) continues on a few remaining issues, but is nearing completion. Work on the revision to the message format specification (RFC 5322bis) has already been completed, provided that no further changes are prompted by changes to RFC 5321bis. Remarkably, both documents have the same editors as the previous two revisions going back to 2001 — John Klensin and Pete Resnick respectively. For those that are interested in what has been corrected and/or clarified in these revisions, each of the documents have appendices that contain a list of changes, with the discussions of these changes having taken place on the working group’s mailing list.

In addition to revising the core specifications, the group is also working on an applicability statement to document other relevant specifications that implementors should be aware of, such as the use of Multipurpose Internet Mail Extensions (MIME) and Transport Layer Security (TLS). It also documents current email best practices, such as which provisions of the transport protocol and message format have proven to cause interoperability issues, and how to properly reuse an existing email message as a template for a new one.

All three documents are expected to be submitted to the RFC Editor for publication in early 2025.