Re: Last Call: draft-resnick-2822upd (Internet Message Format) to Draft Standard

Simon Josefsson <> Fri, 04 April 2008 13:03 UTC

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From: Simon Josefsson <>
To: Brian E Carpenter <>
Subject: Re: Last Call: draft-resnick-2822upd (Internet Message Format) to Draft Standard
In-Reply-To: <> (Brian E. Carpenter's message of "Fri, 04 Apr 2008 13:23:36 +1300")
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Date: Fri, 04 Apr 2008 15:03:32 +0200
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Brian E Carpenter <> writes:

> I am disturbed that the messy situation of X- headers,
> created by RFC 2822's silence on the subject, has
> not been fixed.

Me too.

> I believe it would be appropriate to document that although
> X- headers are widely used, they are not part of the standard
> format and their treatment by Internet MTAs MUST NOT be relied on,


Further, one could discuss that using X- have caused interop problems
when standardizing the header field.  Old applications only know about
the X- form and would not know the non-X- form, which makes it difficult
to standardize the X- header field.  It may be preferable to avoid using
X- in experiment intended to be standardized later on.  This point of
view may be more contentious than what you propose though.  I'd be
interested to understand if others share this opinion.

> unless registered under RFC 3864.

I'd prefer to avoid this escape mechanism.  Can X-* headers really be
registered under RFC 3864?  RFC 822 says:

        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.
     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-">

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