Reference
The email RFCs a developer actually needs
Email runs on a few dozen IETF standards. These are the ones that come up when you're building or debugging a sending path, grouped by job, each with one line on why it matters.
Transport and message format
RFC 5321Simple Mail Transfer Protocol (SMTP)The protocol between mail servers: commands, reply codes, retries.RFC 5322Internet Message FormatHeader syntax: From, To, Date, Message-ID, folding rules.RFC 6409Message Submission for MailPort 587 and how clients submit mail, as opposed to server-to-server relay.RFC 8314Cleartext Considered Obsolete: Use of TLS for Email Submission and AccessWhy implicit TLS on port 465 is back and preferred for submission.RFC 6531SMTP Extension for Internationalized EmailNon-ASCII addresses (SMTPUTF8).
MIME
RFC 2045MIME Part One: Format of Internet Message BodiesContent-Type, Content-Transfer-Encoding, base64 and quoted-printable.RFC 2046MIME Part Two: Media Typesmultipart/alternative and multipart/mixed: how HTML, text and attachments fit together.RFC 2047MIME Part Three: Message Header Extensions for Non-ASCII TextEncoded words for non-ASCII subjects and display names.
Authentication
RFC 7208Sender Policy Framework (SPF)Which servers may send for a domain, and the 10-DNS-lookup limit.RFC 6376DomainKeys Identified Mail (DKIM) SignaturesSigning headers and body so receivers can verify the sending domain.RFC 9989Domain-Based Message Authentication, Reporting, and Conformance (DMARC)Alignment and policy (none, quarantine, reject). Replaced RFC 7489 in May 2026.RFC 9990DMARC Aggregate ReportingThe XML reports receivers send you about mail claiming your domain.RFC 9991DMARC Failure ReportingPer-message failure reports, which few receivers still send.RFC 8601Message Header Field for Indicating Message Authentication StatusThe Authentication-Results header you read when debugging.
Bounces and delivery status
RFC 3463Enhanced Mail System Status CodesThe x.y.z codes like 5.1.1 that tell you why a message bounced.RFC 3464An Extensible Message Format for Delivery Status NotificationsThe structure of a bounce message (DSN).RFC 5965An Extensible Format for Email Feedback ReportsARF, the format behind spam-complaint feedback loops.RFC 3834Recommendations for Automatic Responses to Electronic MailAuto-Submitted and how to avoid auto-reply loops.
Transport security
RFC 8461SMTP MTA Strict Transport Security (MTA-STS)Telling senders to require TLS when delivering to your domain.RFC 8460SMTP TLS ReportingTLS-RPT: daily reports of TLS failures delivering to your domain.RFC 7672SMTP Security via Opportunistic DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS)DNSSEC-based TLS authentication between mail servers.
Links go to the canonical text on rfc-editor.org. Several of these have later updates or errata; check the "Updated by" line there before relying on a detail.