Main Request for Comments (RFC) standards for email protocols and formats

Message Format & Structure
==========================

RFC 5322 – Internet Message Format (structure of email messages, headers like From/To/Subject, and body syntax; obsoletes RFC 2822 and RFC 822)
https://www.rfc-editor.org/rfc/rfc5322.html

RFC 2045–2047, RFC 2231 – MIME (Multipurpose Internet Mail Extensions: defines media types, non-ASCII attachments, and header parameter extensions)
https://www.rfc-editor.org/rfc/rfc2045.html
https://www.rfc-editor.org/rfc/rfc2046.html
https://www.rfc-editor.org/rfc/rfc2047.html
https://www.rfc-editor.org/rfc/rfc2231.html

RFC 6530 / RFC 6531 / RFC 6532 – Internationalized Email (framework, SMTP extension, and headers for non-ASCII/Unicode email addresses)
https://www.rfc-editor.org/rfc/rfc6530.html
https://www.rfc-editor.org/rfc/rfc6531.html
https://www.rfc-editor.org/rfc/rfc6532.html

SMTP (Simple Mail Transfer Protocol)
====================================

RFC 5321 – Simple Mail Transfer Protocol (the core MTA-to-MTA mail transport standard; obsoletes RFC 2821 and RFC 821)
https://www.rfc-editor.org/rfc/rfc5321.html

RFC 6409 – Message Submission for Mail (rules for mail clients submitting messages to a mail server, usually via port 587)
https://www.rfc-editor.org/rfc/rfc6409.html

RFC 1870 – SMTP Service Extension for Message Size Declaration (allows servers/clients to declare maximum message sizes)
https://www.rfc-editor.org/rfc/rfc1870.html

RFC 1652 – SMTP Service Extension for 8bit-MIMEtransport (8BITMIME)
https://www.rfc-editor.org/rfc/rfc1652.html

LMTP (Local Mail Transfer Protocol)
===================================

RFC 2033 – Local Mail Transfer Protocol (a variant of SMTP used by mail delivery agents [MDAs] like Dovecot or Cyrus to deliver mail to a local mailbox without single-recipient bounce complexities)
https://www.rfc-editor.org/rfc/rfc2033.html

POP3 (Post Office Protocol version 3)
=====================================

RFC 1939 – Post Office Protocol - Version 3 (allows a client to download messages from a server and optionally delete them from the server)
https://www.rfc-editor.org/rfc/rfc1939.html

RFC 2449 – POP3 Extension Mechanism (defines capability negotiation for POP3)
https://www.rfc-editor.org/rfc/rfc2449.html

RFC 2595 – Using TLS with IMAP, POP3 and ACAP
https://www.rfc-editor.org/rfc/rfc2595.html

IMAP (Internet Message Access Protocol)
=======================================

RFC 9051 – Internet Message Access Protocol (IMAP) (IMAP4rev2, primary standard for managing messages and folders directly on the server; obsoletes RFC 3501/IMAP4rev1)
https://www.rfc-editor.org/rfc/rfc9051.html

RFC 3501 – INTERNET MESSAGE ACCESS PROTOCOL (predecessor to IMAP4rev2)
https://www.rfc-editor.org/rfc/rfc3501.html

RFC 6851 – IMAP MOVE Extension (adds an atomic MOVE command)
https://www.rfc-editor.org/rfc/rfc6851.html

RFC 6855 – IMAP Support for UTF-8
https://www.rfc-editor.org/rfc/rfc6855.html

RFC 7162 – IMAP Extensions: Quick Flag Changes Resynchronization (CONDSTORE) and QRESYNC
https://www.rfc-editor.org/rfc/rfc7162.html

RFC 7889 – The IMAP APPENDLIMIT Extension (advertises maximum upload size)
https://www.rfc-editor.org/rfc/rfc7889.html

JMAP (JSON-based Alternative to IMAP)
=====================================

RFC 8620 – The JSON Meta Application Protocol (JMAP) for Mail (modern HTTP/JSON API for mail retrieval and management)
https://www.rfc-editor.org/rfc/rfc8620.html

RFC 8621 – The JSON Meta Application Protocol (JMAP) Data Types for Mail
https://www.rfc-editor.org/rfc/rfc8621.html

Email Security, Authentication & Filtering
==========================================

RFC 6376 & RFC 7208 – DKIM and SPF (cryptographic signature verification and sender policy framework via DNS)
https://www.rfc-editor.org/rfc/rfc6376.html

RFC 7489 & RFC 8617 – DMARC and ARC (policy enforcement combining SPF/DKIM, and chain of custody for forwarded emails)
https://www.rfc-editor.org/rfc/rfc7489.html

RFC 3207 & RFC 8314 – Transport Layer Security (STARTTLS extensions and obsolete cleartext guidelines)
https://www.rfc-editor.org/rfc/rfc3207.html

RFC 5228 – Sieve (an email filtering language)
https://www.rfc-editor.org/rfc/rfc5228.html

