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Re: [ietf-smtp] Fwd: [dispatch] Forced SMTP redirects

2021-03-16 10:27:06
On 7/9/2020 9:01 PM, John C Klensin wrote:
Also remember that SMTP encourages, nay requires, that if an
SMTP-sender gets back a reply code it does not recognize, it is
required to ignore anything but the first digit.  That predicts
a fairly hard road at this point for new codes that are intended
to convey subtle information.

Recent list discussion prompted me to look back for earlier reference to the first digit 'rule', concerning test in Section 4.2 of RFC 5321.

The above logic escapes me.

While it gives direction when the full code is not (yet) understood, it says nothing about adoption processes that might or might not eventually lead to support for new codes.

That is, new codes will of course not be understood by any implementation initially, but useful codes are likely to be understood by 'most' implementations eventually.

As such, the first digit rule, in RFC 5321, really has nothing at all to do with the efficacy of adding code or adding text to codes. It only has to do with behavior of implementations that do not (yet) understand a full code.


Dave Crocker
Brandenburg InternetWorking

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