RFCs for incoming email, mailboxes, and filtering.
The RFCs below are useful starting points for understanding the standards behind inbound SMTP, MX routing, IMAP, POP3, authentication checks, abuse reporting, MIME, and common filtering signals.
SMTP, MX, and incoming delivery
Core transport and DNS documents for moving messages between mail servers and finding the right receiving host.
RFC 5321
Simple Mail Transfer Protocol
RFC 5322
Internet Message Format
RFC 1035
Domain Names - Implementation and Specification
RFC 974
Mail Routing and the Domain System
RFC 3207
SMTP Service Extension for Secure SMTP over TLS
RFC 3463
Enhanced Mail System Status Codes
RFC 3464
Delivery Status Notifications
IMAP and mailbox access
Specifications for server-side mailboxes, message flags, folders, synchronisation, and client access.
RFC 9051
Internet Message Access Protocol - Version 4rev2
RFC 3501
Internet Message Access Protocol - Version 4rev1
RFC 2177
IMAP4 IDLE command
RFC 6851
IMAP MOVE Extension
RFC 4315
IMAP UIDPLUS Extension
POP3 retrieval
Documents for simple mailbox download, POP3 extensions, service discovery, and modern TLS expectations.
RFC 1939
Post Office Protocol - Version 3
RFC 2449
POP3 Extension Mechanism
RFC 6186
Use of SRV Records for Locating Email Submission and Access Services
RFC 8314
Cleartext Considered Obsolete for Email Submission and Access
Authentication and filtering signals
Common standards used by receiving systems when evaluating trust, alignment, authentication, and reporting.
RFC 7208
Sender Policy Framework (SPF)
RFC 6376
DomainKeys Identified Mail (DKIM) Signatures
RFC 7489
Domain-based Message Authentication, Reporting, and Conformance (DMARC)
RFC 8601
Authentication-Results Message Header Field
RFC 8617
Authenticated Received Chain (ARC)
RFC 5965
Abuse Reporting Format
Message content and mailbox handling
Related documents for message bodies, MIME structure, list headers, and server-side filtering rules.
RFC 2045
MIME Part One: Format of Internet Message Bodies
RFC 2046
MIME Part Two: Media Types
RFC 2369
Use of URLs as Meta-Syntax for Core Mail List Commands
RFC 8058
Signaling One-Click Functionality for List Email Headers
RFC 5228
Sieve: An Email Filtering Language