Simple Mail Transfer Protocol
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J. Klensin · 2008-10
Abstract
RFC 5321 defines the current authoritative specification of the Simple Mail Transfer Protocol (SMTP), updating RFC 2821. It specifies the commands, responses, and operational procedures for mail transfer between SMTP servers, including the extended SMTP (ESMTP) framework, internationalized addresses considerations, and updated guidance on relay behavior, error handling, and message size limits.
Why This RFC Matters
RFC 5321 is the definitive SMTP specification used by all modern mail transfer agents (MTAs) including Postfix, Exim, Sendmail, and Microsoft Exchange. Its clarifications on relay policies, bounce handling, and address verification helped mail system operators combat spam more effectively. The RFC also refined the EHLO/ESMTP negotiation framework that allows email servers to advertise and negotiate capabilities — the mechanism used by STARTTLS, SIZE declarations, and authentication extensions. Together with SPF (RFC 7208), DKIM (RFC 6376), and DMARC (RFC 7489), RFC 5321 forms the foundation of authenticated Internet email.