Reference catalog of 68 IETF RFCs that define how email works. Each entry links to a full identity card with the published date, domain, SMTP relevance, evolution chain, and a direct link to the IETF spec. Tiered by practical importance for senders, developers, and deliverability practitioners.
These 22 RFCs define the core protocols that every MTA, ESP, and deliverability stack implements. If you send, receive, or troubleshoot email professionally, these are the standards to know.
The authoritative SMTP standard defining the full MTA-to-MTA command sequence. Every mail server on the internet implements this RFC.
Defines the structure of email messages: headers (From, To, Subject, Date), body format, and field syntax. The rulebook for every email message.
Cryptographic signature standard that lets receiving MTAs verify a message was authorised by the sending domain. Required by Gmail and Yahoo since Feb 2024.
Defines which IP addresses are authorised to send mail for a domain via DNS TXT records. Required for bulk senders since Feb 2024.
Ties SPF and DKIM together into a single policy record. Instructs receivers how to handle failing messages and enables aggregate reporting.
Adds a chain of authentication headers so intermediate forwarders (mailing lists, aliases) can preserve SPF/DKIM/DMARC status across hops.
The five MIME RFCs as a suite: message body format, media types, header extensions, registration procedures, and conformance. Foundation for attachments and HTML email.
The current TLS standard. Mandatory reading for secure SMTP (STARTTLS/SMTPS) and IMAP/POP3 over TLS. Supersedes TLS 1.2 (RFC 5246).
Mandates TLS for SMTP submission (port 465/587), IMAP, and POP3. Declares cleartext email protocols obsolete. The compliance baseline for modern mail infrastructure.
Defines the STARTTLS SMTP extension that upgrades a plain-text connection to TLS in-band. The mechanism behind opportunistic TLS on port 25.
Defines the SMTP AUTH command and the SASL framework used to authenticate SMTP clients to submission servers. Required for any authenticated mail submission.
Defines port 587 as the mail submission port, separating client-to-server submission from MTA-to-MTA relay on port 25. The basis of modern email sending infrastructure.
Defines the List-Unsubscribe-Post header enabling one-click unsubscribe. Required by Google and Yahoo bulk sender rules since February 2024 for high-volume senders.
Formalises the greylisting technique where MTAs temporarily reject unknown senders to filter out botnet spam. Explains why legitimate mail is sometimes delayed on first send.
Defines the Null MX (0 .) DNS record that explicitly signals a domain accepts no incoming mail, preventing delivery loops to non-mail domains.
Defines the machine-readable DSN format used in bounce messages. The structured format behind every hard bounce your ESP reports.
Defines standard role mailboxes every domain should maintain: postmaster@, abuse@, hostmaster@, noc@. Required for RFC 5321 compliance and blocklist removal requests.
Defines the POP3 protocol for downloading email from a server to a local client. Still the retrieval protocol used by millions of email clients.
The current IMAP standard for online mailbox access, folder management, and real-time message retrieval. The protocol behind every modern email client’s sync.
Defines the MIME content-type header and the framework for non-ASCII message bodies. The foundation of HTML email and attachments.
Defines the media type system: text, image, audio, video, application, multipart, message. The classification system used by every email client and spam filter.
Defines encoded-word syntax for non-ASCII characters in headers. Behind every accented character, emoji, or CJK glyph in a Subject: or From: field.
Detailed specifications for SMTP extensions, MIME subsystems, IMAP extensions, authentication details, mailing lists, internationalization, and message format standards. Essential for implementers and advanced practitioners.
Conceptual overview of the entire internet mail system: MSA, MTA, MDA, MUA roles and their interactions.
Defines the EHLO command and the ESMTP extension framework that all modern SMTP servers use.
Lets a sending MTA declare message size upfront so the receiver can reject oversized mail before transfer.
Defines the 8BITMIME SMTP extension, allowing 8-bit message content without base64 encoding.
Defines the Authentication-Results header that records SPF, DKIM, and DMARC check outcomes. Used by BIMI and downstream filtering.
Operational guide for deploying DKIM: key rotation, selector management, DNS setup, and troubleshooting.
Third-party endorsement mechanism where a trusted voucher certifies a sender’s reputation via DNS.
Defines the ARF (Abuse Reporting Format) used in Feedback Loop complaint reports from mailbox providers.
Defines the machine-readable DSN format used in bounce messages and delivery receipts.
Defines MDN (read receipt) format: the message a mail client sends when a message is read, deleted, or processed.
Defines the multipart/report MIME type used to structure delivery status notifications and administrative messages.
Registers the full set of standard email header fields with IANA. The canonical header field reference.
Defines List-Unsubscribe, List-Help, List-Post and other headers for automating mailing list management.
Defines the List-Id header for uniquely identifying mailing lists, used by email clients to organise list mail.
Defines MIME conformance levels and provides worked examples for implementers.
Defines charset and language tagging for MIME parameter values, used in Content-Disposition filenames with non-ASCII characters.
Defines Content-Disposition that controls whether attachments are shown inline or as file downloads.
Defines cid: and mid: URI schemes for referencing MIME parts within a message, used in HTML emails with embedded images.
Defines MIME framework for signed and encrypted message parts, foundation for S/MIME and PGP/MIME.
Defines S/MIME 3.2 for signing and encrypting email using X.509 certificates.
Defines the IDLE extension that lets an IMAP client be notified of new mail in real time without polling.
Defines CONDSTORE, atomic flag updates with change detection to prevent race conditions in concurrent IMAP clients.
Defines server-side message sorting and threading extensions for IMAP, reducing client-side processing.
Defines the MOVE command for atomically moving messages between IMAP folders without a COPY+EXPUNGE sequence.
Extends IMAP URLFETCH to support binary and format-converted part retrieval.
Defines the CAPA command that lets POP3 clients discover server capabilities before authenticating.
Framework RFC for EAI (Email Address Internationalization) supporting Unicode email addresses and headers.
Defines the SMTPUTF8 SMTP extension that allows UTF-8 in MAIL FROM, RCPT TO, and headers.
Defines UTF-8 encoded headers for internationalized email messages.
Defines how international domain names are encoded for use in DNS and email addresses.
Practical guide for validating email addresses, URLs, and domain names in applications.
Defines the Content-Language header for indicating the natural language of message content.
Registers the application/mbox MIME type for the traditional Unix mbox mailbox format.
These RFCs have been formally obsoleted by newer standards. They are included here for historical context, debugging legacy systems, and understanding the evolution of each protocol. Do not implement these for new systems.
Previous SMTP standard. Introduced port 587 and clarified relay rules. Superseded by RFC 5321 in 2008.
Previous email message format standard. Superseded by RFC 5322 in 2008.
Original SPF specification. Obsoleted by RFC 7208 in 2014 with stricter language and clarifications.
Original DKIM specification. Obsoleted by RFC 6376 in 2011.
Defined TLS 1.2. Obsoleted by RFC 8446 (TLS 1.3). Still widely deployed but being phased out.
Early TLS-for-email RFC. Obsoleted by RFC 8314 which mandates implicit TLS over the legacy STARTTLS approach.
Earlier version of the submission RFC. Obsoleted by RFC 6409 in 2011.
Previous MDN RFC. Obsoleted by RFC 8098 in 2017.
Original MIME type registration procedures. Obsoleted by RFC 4288.
The original email message format standard from 1982. Historical foundation of all subsequent format RFCs.
Original SMTP RFC from 1982 by Jon Postel. Defined the core SMTP command set still in use today.
The earliest email transfer protocol RFC, predecessor to SMTP. Published September 1980.
Defines the AUTH command for POP3. Obsoleted by RFC 5034.
Extends DSN and MDN formats for internationalized addresses. Obsoleted by RFC 6533.
About the Author
Alaa · LinkedIn
Email infrastructure specialist with 8+ years of hands-on experience in SMTP, deliverability, and email verification. I’ve configured and troubleshot mail systems across Postfix, Exchange, and cloud relays, managed IP reputation and warmup campaigns, and built verification pipelines processing millions of addresses. My work spans DNS authentication (SPF, DKIM, DMARC, BIMI), bounce handling, blocklist monitoring, and compliance frameworks including CAN-SPAM and GDPR. I write every article on SMTPedia to give email professionals, developers, and marketers the accurate, RFC-grounded reference they need.
About SMTPedia
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