Simple Mail Transfer Protocol
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J. Klensin · 2001-04
Abstract
RFC 2821 defines the Simple Mail Transfer Protocol (SMTP), the standard for electronic mail transmission across the Internet. SMTP operates over TCP port 25 (or 587 for submission) and specifies the commands and responses for establishing connections, transferring mail envelopes and message content, and handling errors. It updated and clarified the original SMTP specification in RFC 821.
Why This RFC Matters
SMTP is the backbone of Internet email, responsible for routing and delivering billions of messages daily between mail servers worldwide. RFC 2821 updated and clarified RFC 821, incorporating two decades of operational experience including the introduction of ESMTP extensions (RFC 1869), the STARTTLS upgrade mechanism, and anti-relay policies developed in response to spam. Email authentication protocols SPF (RFC 7208), DKIM (RFC 6376), and DMARC (RFC 7489) were all built on top of the SMTP infrastructure defined here. RFC 2821 was itself superseded by RFC 5321.