Simple Mail Transfer Protocol (SMTP) is the foundational internet standard for transmitting electronic mail across networks. Defined in RFC 5321 (October 2008), SMTP uses a command-response model over TCP with a comprehensive set of commands (EHLO, MAIL FROM, RCPT TO, DATA, QUIT) and numeric response codes. Works in conjunction with RFC 5322 (Internet Message Format) for message structure.
| Standard | Description |
|---|---|
| RFC 5321 | Simple Mail Transfer Protocol (primary specification) |
| RFC 5322 | Internet Message Format (email message structure) |
- smtp-message-schema.json — Email message schema (RFC 5321/5322)
- smtp-session-schema.json — SMTP session and commands schema
- smtp-message-structure.json — Email message object structure
- smtp-context.jsonld — Linked data context mapping SMTP concepts to schema.org
- smtp-session-example.json — Complete SMTP session command/response flow
- smtp-message-example.json — RFC 5322 message with MIME attachment
- smtp-vocabulary.yml — Complete SMTP vocabulary: commands, response codes, extensions, ports
Standard Ports:
- Port 25: Server-to-server transfer (MTA to MTA)
- Port 587: Client submission (MUA to MTA, authentication required)
- Port 465: SMTP over TLS (implicit)
Key ESMTP Extensions: STARTTLS, AUTH, SIZE, 8BITMIME, PIPELINING, SMTPUTF8, DSN