Re: [ietf-smtp] Is this a valid Message-ID header ?

John C Klensin <> Fri, 06 March 2020 05:31 UTC

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Date: Fri, 06 Mar 2020 00:31:19 -0500
From: John C Klensin <>
To: John R Levine <>, Mark Andrews <>
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Subject: Re: [ietf-smtp] Is this a valid Message-ID header ?
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--On Thursday, March 5, 2020 23:40 -0500 John R Levine
<> wrote:

> On Fri, 6 Mar 2020, Mark Andrews wrote:
>> Conversion of all ASCII headers to utf-8 mime encoded should
>> be banned. Apart from testing encoding/decoding it serves no
>> useful purpose and if you want to perform such testing you
>> can always add a non ASCII character.  MUA's that do this
>> are deliberately breaking interoperability with old MUAs.
> The spec is pretty clear where you can have MIME entities and
> where you can't.  You can't have them in Message-ID's,
> References, or the actual address in To, From, Cc, etc.

In the interest of clarity, what you are referring to as "MIME
entities" are encoded words as specified in RFC 2231.  Right?

And because encoded words are not permitted in Message-IDs or
the other fields you mention, those fields must conform to the
specified syntax without decoding.   If they don't, it is pretty
much up to the receiving system what to do about it.  I suppose
that could including treating the strings as encoded works and
see where decoding takes one, but that would be so far beyond
the standard that questions about the decoded (UTF-8 or
otherwise) field looks like are really not relevant here: such a
string is either valid or invalid without decoding and, as far
as the standard is concerned, that is the end of it.