From: Greg A. Woods (woodsweird.com)
Date: Sat May 17 2003 - 14:03:34 CDT

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[ On Saturday, May 17, 2003 at 18:22:55 (+0200), Ralf Hildebrandt wrote: ]
> Subject: List of non-experimental email headers?
>
> I know that headers starting with X- are considered experimental, but
> where can the authoritative list of NON-experimenatl header be found?

Note that "X-" does not mean "experimental", but rather "extension".

Please refer to RFC 822:

     extension-field =
                   <Any field which is defined in a document
                    published as a formal extension to this
                    specification; none will have names beginning
                    with the string "X-">

     4.7.4. EXTENSION-FIELD

             A limited number of common fields have been defined in
        this document. As network mail requirements dictate, addi-
        tional 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-".

             Names of Extension-fields are registered with the Network
        Information Center, SRI International, Menlo Park, California.

Note also that _ANY_ unused field header is valid as a user defined
field, as is noted in RFC 822:

     user-defined-field =
                   <Any field which has not been defined
                    in this specification or published as an
                    extension to this specification; names for
                    such fields must be unique and may be
                    pre-empted by published extensions>

     4.7.5. USER-DEFINED-FIELD

             Individual users of network mail are free to define and
        use additional header fields. Such fields must have names
        which are not already used in the current specification or in
        any definitions of extension-fields, and the overall syntax of
        these user-defined-fields must conform to this specification's
        rules for delimiting and folding fields. Due to the
        extension-field publishing process, the name of a user-
        defined-field may be pre-empted

        Note: The prefatory string "X-" will never be used in the
               names of Extension-fields. This provides user-defined
               fields with a protected set of names.

Sadly yet another very significant flaw in the new RFC 2822 is that it
does not seem to carry forward these definitions.

Note also that if there were a canonical list of message header field
names then it would probably be maintained by IANA, and would not
necessarily be given in any given RFC.

--
                                                                Greg A. Woods

+1 416 218-0098; <g.a.woodsieee.org>; <woodsrobohack.ca>
Planix, Inc. <woodsplanix.com>; VE3TCP; Secrets of the Weird <woodsweird.com>

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