Quick reference to Internet message headers

The following table lists Internet message header fields which have been defined in RFCs or actually used or both, in E-mail messages, Usenet messages, or other contexts. For each header, its name, a short description, and some reference is given; the reference tries to point to a document where the meaning of the header is specified or described. In several cases, the description is just an educated (?) guess. The use of the word nonstandard here typically means just lack of more specific information; it does not imply that other headers would necessarily be standard. This is a descriptive document, aimed at helping to find information in official and unofficial sources. This document does not cover HTTP headers; see a separate Quick reference to HTTP headers.

The alphabetic order is by words, counting a hyphen as word separator, so e.g. Mail-System-Version appears before Mailer.

Notes on header names in general:

• By RFC 822 clause 3.4.7, header names are case-insensitive. The spellings above try to correspond to those used in the specifications. For example, RFC 822 writes cc, though in practice Cc or CC has become more common; they are all equivalent of course.
• In RFC 822 clause 4.7.4 says: "A limited number of common fields have been defined in this document. As network mail requirements dictate, additional fields may be standardized. To provide user-defined fields with a measure of safety, in name selection, such extension-fields will never have names that begin with the string X-." It also specifies a registration authority for extension fields, assumably meaning that all header field names not beginning with X- should be registered. In practice, there doesn't seem to have been any central registry; and headers for private, experimental, or even public use have been named using or not using X- prefix in a rather random fashion. See the proposal Mail and Netnews Header field Registration Procedure (expired Internet-Draft) by Jacob Palme for an explanation of what problems have been caused by this and how they might be solved.

There is surely a much larger set of X- headers used somewhere than the list above tells. It could be some sort of sports to add headers just for fun, on user agents that allow arbitrary headers to be inserted. Examples:
X-Anagram: look vanilla, aim elite
X-Eric-Conspiracy: there is no conspiracy

Explanations of abbreviations:

MIME

Multipurpose Internet Mail Extensions, a set of specifications for extending the format of Internet E-mail to allow non-US-ASCII textual messages, non-textual messages, multipart message bodies, and non-US-ASCII information in message headers; used outside E-mail too

MTA

Message Transfer Agent, generically a program responsible for delivering E-mail messages

PICS

Platform for Internet Content Selection, a method for associating metadata (PICS labels) to be associated with Internet content, so that users can filter content; originally designed to help parents and teachers control what children access on the Internet.