Re: [ietf-smtp] SMTP Reply code 500

John C Klensin <john-ietf@jck.com> Wed, 04 March 2020 14:57 UTC

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Date: Wed, 04 Mar 2020 09:57:34 -0500
From: John C Klensin <john-ietf@jck.com>
To: Paul Smith <paul@pscs.co.uk>, ietf-smtp@ietf.org
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Subject: Re: [ietf-smtp] SMTP Reply code 500
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--On Wednesday, March 4, 2020 09:33 +0000 Paul Smith
<paul@pscs.co.uk> wrote:

> A buggy SMTP server could send an invalid response to the SMTP
> client  (ie a response not starting with a three digit number
> beginning with 1 to 5). In that case, the SMTP client should
> do whatever it wants to do, ranging from ignoring it, to
> dropping the connection in disgust and reporting an error to
> the user/administrator/log/whatever (I'd probably advocate for
> the latter).

In the interest of cleanliness (i.e., the fact that they are
breaking the rules doesn't mean I should, even if I'm
disgusted), I would probably advocate for sending a QUIT command
prior to dropping the connection, but yes.

Certainly trying to continue a session with a server that is
badly implemented enough, or has gotten confused enough, to send
responses that don't start with a digit in the 1-5 range would
be ill-advised.

   john