RFC 5321: Simple Mail Transfer Protocol
The authoritative SMTP standard defining the full MTA-to-MTA command sequence.
RFCs
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 772: Mail Transfer Protocol
The earliest email transfer protocol RFC, predecessor to SMTP.
RFC 2821: Simple Mail Transfer Protocol
Previous SMTP standard. Introduced port 587 and clarified relay rules. Obsoleted by RFC 5321 in 2008.
RFC 1870: SMTP Service Extension for Message Size Declaration
Defines the SIZE extension that lets a sending MTA declare message size upfront so the receiver can reject oversized mail before transfer.
RFC 1869: SMTP Service Extensions (ESMTP)
Defines the EHLO command and the ESMTP extension framework that all modern SMTP servers use.
RFC 6152: SMTP Service Extension for 8-bit MIME Transport
Defines the 8BITMIME SMTP extension, allowing 8-bit message content without base64 encoding.
RFC 3207: SMTP Service Extension for Secure SMTP over TLS (STARTTLS)
Defines the STARTTLS SMTP extension that upgrades a plain-text SMTP connection to TLS in-band.
RFC 4954: SMTP Service Extension for Authentication
Defines the SMTP AUTH command and the SASL framework used to authenticate SMTP clients to submission servers.
RFC 2045: MIME Part One: Format of Internet Message Bodies
Defines the MIME content-type header and the framework for non-ASCII message bodies.
RFC 2046: MIME Part Two: Media Types
Defines the media type system (text, image, audio, video, application, multipart, message).
RFC 2047: MIME Part Three: Message Header Extensions for Non-ASCII Text
Defines encoded-word syntax for non-ASCII characters in headers.