Registration of Mail and MIME Header Fields
updated by
- rfc-5322 — Internet Message Format
Extracted elements (28)
Several RFC 1327 header fields (Content-Return, Content-Identifier, Expiry-Date, Obsoletes) were subsequently renamed in RFC 2156 to avoid confusion with MIME-defined fields. The old names carry 'obsolete' status while the renamed versions remain standards-track.
The Content-Base header field was included in the first version of MHTML (RFC 2110) and HTTP 1.1 but was removed in the second version (RFC 2557). Implementations using Content-Location should be preferred for resolving relative URIs within a content part.
The Encrypted header field, originally defined by RFC 822, was found to be inadequately specified, was not widely implemented, and was removed in RFC 2822. Current practice is to use separate encryption such as S/MIME or OpenPGP, possibly with RFC 1847 MIME security multiparts.
The Resent-Reply-To header field was defined by RFC 822 but was obsoleted by RFC 2822. Implementations should not generate it, though they may encounter it in legacy messages.
The Accept-Language header field indicates a language the message sender requests for auto-responses. Noted problems include determining which email address it refers to, internationalization costs, language subtag interpretation, and character set encoding selection.
The Autosubmitted and Autoforwarded header fields indicate that a message has been automatically submitted or automatically forwarded, respectively. Both are RFC 2156 (MIXER) fields not intended for general use.
The Bcc header field contains addresses of recipients whose addresses are not to be revealed to other recipients of the message. Defined as standard by RFC 822; specified in RFC 2822 section 3.6.3.
The Content-Disposition header field indicates whether a MIME body part is to be shown inline or is an attachment, and can suggest a filename for saving an attachment. Specified in RFC 2183.
The Content-Duration header field specifies the time duration of a MIME body part's content in seconds, useful for audio or video messages. Specified in RFC 2424 (updated by RFC 3803).
The Content-MD5 header field carries an MD5 checksum of the MIME body part content to ensure it has not been modified in transit. Specified in RFC 1864.
The Content-Transfer-Encoding header field specifies the coding method used in a MIME message body part. Specified in RFC 2045 section 6.
The Content-Type header field specifies the format of the MIME message body part (character set, media type, etc.). The 'charset' parameter is mandatory for all textual Content-Types. The MIME definition (RFC 2045) should be used in generating mail; RFC 1049 has 'historic' status.
The Date header field specifies the date and time at which the creator of the message indicated it was complete and ready to enter the mail delivery system. Defined as standard by RFC 822; specified in RFC 2822 section 3.6.1.
The Disclose-Recipients header field, primarily an X.400 facility, tells whether recipients are to be informed of the names of other recipients. In Internet mail, recipient disclosure is done via To:, Cc:, and Bcc: header fields. RFC 2156 (MIXER); not for general use.
The Disposition-Notification-To header field indicates that the sender wants a disposition notification (read receipt) when the message is received or processed by its recipients. Specified in RFC 2298 (updated by RFC 3798).
The From header field specifies the author(s) of the message — the mailbox(es) of the person(s) or system(s) responsible for writing it. Defined as standard by RFC 822; specified in RFC 2822 section 3.6.2.
The Importance header field is a hint from the originator to recipients about message importance, with values High, normal, or low. It is not used to control transmission speed. Proposed for use with RFC 2156 (MIXER) and RFC 3801 (VPIM).
The List-* header fields (List-Archive, List-Help, List-ID, List-Owner, List-Post, List-Subscribe, List-Unsubscribe) provide URLs for mailing list management operations and identification. Defined in RFC 2369 and RFC 2919.
The Message-Context header field provides information about the context and presentation characteristics of a message, with values including 'voice-message', 'fax-message', 'pager-message', 'multimedia-message', 'text-message', or 'none'. Specified in RFC 3458.
The Message-ID header field contains a single unique message identifier referring to a particular version of a message. If the message is resent without changes, the original Message-ID is retained. Defined as standard by RFC 822; specified in RFC 2822 section 3.6.4.
The MIME-Version header field indicates that the message is formatted according to the MIME standard and specifies which version of MIME is used. Specified in RFC 2045 section 4.
The Received header field contains information about receipt of the current message by a mail transfer agent on the transfer path, forming the mail transfer trace. Defined as standard by RFC 822; specified in RFC 2822 section 3.6.7 and RFC 2821.
The References header field contains message identifiers of other messages to which the current message is related. RFC 2822 changed the definition to require a list of all Message-IDs in the preceding reply chain. Specified in RFC 2822 section 3.6.4.
The Return-Path header field provides the return path for message response diagnostics. It is added by the final MTA; see also RFC 2821. Defined as standard by RFC 822; specified in RFC 2822 section 3.6.7.
The Sender header field specifies the mailbox of the agent responsible for the actual transmission of the message, distinct from the author(s) listed in From. Defined as standard by RFC 822; specified in RFC 2822 section 3.6.2.
The Sensitivity header field indicates how sensitive it is to disclose the message to people other than specified recipients, with values Personal, private, and company-confidential. Absence in X.400-gatewayed messages indicates the message is not sensitive.
RFC 4021 establishes the initial IANA 'Permanent Message Header Field Registry' entries for mail and MIME header fields, per the registration procedures defined in RFC 3864 (BCP 90). The registry covers 75 mail header fields and 13 MIME header fields.
No security considerations are introduced by this registration document beyond those already inherent in use of the mail message header fields referenced in the individual specifications.