RFC 8314: Cleartext Considered Obsolete: Use of TLS for Email Submission and Access
Mandates TLS for SMTP submission (port 465/587), IMAP, and POP3. Declares cleartext email protocols obsolete.
RFCs
RFC 2049: MIME Part Five: Conformance Criteria and Examples
Defines MIME conformance levels and provides worked examples for implementers.
RFC 2231: MIME Parameter Value and Encoded Word Extensions
Defines charset and language tagging for MIME parameter values, used in Content-Disposition filenames with non-ASCII characters.
RFC 7208: Sender Policy Framework (SPF)
Defines which IP addresses are authorised to send mail for a domain via DNS TXT records.
RFC 4408: Sender Policy Framework (SPF)
Original SPF specification. Obsoleted by RFC 7208 in 2014 with stricter language and clarifications.
RFC 6376: DomainKeys Identified Mail (DKIM) Signatures
Cryptographic signature standard that lets receiving MTAs verify a message was authorised by the sending domain.
RFC 4871: DomainKeys Identified Mail (DKIM) Signatures
Original DKIM specification. Obsoleted by RFC 6376 in 2011.
RFC 2048: MIME Part Four: Registration Procedures
Original MIME type registration procedures. Obsoleted by RFC 4288.
Email RFC Directory: 68 Standards Explained
Reference catalog of 68 email protocol RFCs. Tiered by importance: essential standards every sender must know, complete reference catalog, and historical archive.
RFC 5322: Internet Message Format
Defines the structure of email messages: headers, body, encoding, and field syntax.
RFC 2822: Internet Message Format
Previous email message format standard. Obsoleted by RFC 5322 in 2008.
RFC 822: Standard for ARPA Internet Text Messages
The original email message format standard from 1982. Historical foundation of all subsequent format RFCs.