RFC 5598: Internet Mail Architecture
Conceptual overview of the entire internet mail system: MSA, MTA, MDA, MUA roles and their interactions.
Internet Standards
RFC 2392: Content-ID and Message-ID Uniform Resource Locators
Defines cid: and mid: URI schemes for referencing MIME parts within a message, used in HTML emails with embedded images.
RFC 1939: Post Office Protocol Version 3 (POP3)
Defines the POP3 protocol for downloading email from a server to a local client.
RFC 2048: MIME Part Four: Registration Procedures
Original MIME type registration procedures. Obsoleted by RFC 4288.
RFC 2822: Internet Message Format
Previous email message format standard. Obsoleted by RFC 5322 in 2008.
RFC 5321: Simple Mail Transfer Protocol
The authoritative SMTP standard defining the full MTA-to-MTA command sequence.
RFC 821: Simple Mail Transfer Protocol
Original SMTP RFC from 1982 by Jon Postel. Defined the core SMTP command set still in use today.
RFC 2821: Simple Mail Transfer Protocol
Previous SMTP standard. Introduced port 587 and clarified relay rules. Obsoleted by RFC 5321 in 2008.
