RFC 4021: Registration of Mail and MIME Header Fields
Registers the full set of standard email header fields with IANA, providing a canonical reference.
Internet protocols
RFC 3282: Content Language Headers
Defines the Content-Language header for indicating the natural language of message content.
RFC 5322: Internet Message Format
Defines the structure of email messages: headers, body, encoding, and field syntax.
RFC 822: Standard for ARPA Internet Text Messages
The original email message format standard from 1982. Historical foundation of all subsequent format RFCs.
RFC 2046: MIME Part Two: Media Types
Defines the media type system (text, image, audio, video, application, multipart, message).
