Email standards

Email structure

These RFCs define the way emails themselves are structured.

RFC 5322 — Internet Message Format (basic format of an email message), previously RFC 822 and RFC 2822.

RFC 2045 — Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies. This is an extension to the email message format to support attachments and non-ASCII data in emails.

RFC 2046 — Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types.

RFC 2047 — MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text.

RFC 2231 — MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations.

Email protocols

These RFCs define how emails are transported between computers, both for sending (SMTP) and receiving (IMAP/POP).

RFC 5321 — Simple Mail Transfer Protocol. This is a protocol used to send emails between computers. This was previously RFC 821 and RFC 2821.

RFC 3501 — INTERNET MESSAGE ACCESS PROTOCOL — VERSION 4rev1. This is the IMAP protocol, used to read emails.

RFC 4551 — IMAP Extension for Conditional STORE Operation or Quick Flag Changes Resynchronization. This is an IMAP extension that adds MODSEQ as a way to quickly find changes to a mailbox.

RFC 1939 — Post Office Protocol, Version 3. This is the older POP protocol, used to read emails.

RFC 8620 — The JSON Meta Application Protocol (JMAP). This is a new protocol for fetching, organizing, and sending messages, to replace older IMAP and SMTP protocols.

Email security

These RFCs define some security standards for email protocols and formats.

RFC 2595 — Using TLS with IMAP, POP3 and ACAP. This is a protocol used to upgrade a plaintext IMAP/POP connection to an SSL/TLS encrypted one.

RFC 3207 — SMTP Service Extension for Secure SMTP over Transport Layer Security. This is a protocol used to upgrade a plaintext SMTP connection to an SSL/TLS encrypted one.

RFC 5246 — The Transport Layer Security (TLS) Protocol Version 1.2. This is a protocol used to encrypt a connection.

RFC 6376 — DomainKeys Identified Mail (DKIM) Signatures. This allows emails to be signed by a particular domain to ensure they haven't been tampered with, and to say that that domain claims responsibility for the message.

RFC 8617 — Authenticated Received Chain (ARC). This is a protocol to provide an authenticated chain of custody for messages that have passed through intermediate mail servers, such as through forwarding.

RFC 4408 — Sender Policy Framework (SPF) for Authorizing Use of Domains in E-Mail