RFC 6647: Email Greylisting: An Applicability Statement for SMTP
Formalises the greylisting technique where MTAs temporarily reject unknown senders to filter out botnet spam.
smtp
RFC 6531: SMTP Extension for Internationalized Email
Defines the SMTPUTF8 SMTP extension that allows UTF-8 in MAIL FROM, RCPT TO, and headers.
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 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 4954: SMTP Service Extension for Authentication
Defines the SMTP AUTH command and the SASL framework used to authenticate SMTP clients to submission servers.
How to Add Smtp.com Unsubscribe Link: Step-By-Step Guide
Learn how to easily add an SMTP.com unsubscribe link to your emails to boost compliance and user trust—discover the essential steps inside.
Email Protocols and Ports in 2026: SMTP, IMAP, POP3, OAuth2 Requirements and TLS Configuration
SMTP, IMAP, and POP3 protocols and ports for email: 25/465/587 for SMTP, 993 for IMAP, 995 for POP3. Covers STARTTLS vs implicit TLS, OAuth2 requirements for Gmail Workspace and Microsoft 365, POP3 ...
