RFC 4021: Registration of Mail and MIME Header Fields
Registers the full set of standard email header fields with IANA, providing a canonical reference.
Email standards
RFC 5337: Internationalized Delivery Status and Disposition Notifications
Extends DSN and MDN formats to support internationalized addresses.
RFC 4155: The Application/Mbox Media Type
Registers the application/mbox MIME type for the traditional Unix mbox mailbox format.
RFC 2045-2049: MIME (RFC 2045 through 2049): Attachments and HTML Emails
Overview article covering all five MIME RFCs as a suite: format, media types, header extensions, registration, and conformance.
RFC 6409: Message Submission for Mail
Defines port 587 as the mail submission port, separating client-to-server submission from MTA-to-MTA relay on port 25.
RFC 4409: Message Submission for Mail
Earlier version of the submission RFC. Obsoleted by RFC 6409.
RFC 8098: Message Disposition Notification
Defines MDN (read receipt) format: the message a mail client sends when a message is read, deleted, or processed.
RFC 3798: Message Disposition Notification
Previous MDN RFC. Obsoleted by RFC 8098 in 2017.
RFC 3282: Content Language Headers
Defines the Content-Language header for indicating the natural language of message content.
RFC 2919: List-Id: A Structured Field and Namespace for the Identification of Mailing Lists
Defines the List-Id header for uniquely identifying mailing lists, used by email clients to organise list mail.
