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Re: [ietf-smtp] SMTP Reply code 500

2020-03-04 20:39:54
On Wed, 2020-03-04 at 09:57 -0500, John C Klensin wrote:
--On Wednesday, March 4, 2020 09:33 +0000 Paul Smith
<paul(_at_)pscs(_dot_)co(_dot_)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.



Thank you very much for the reply. 
My doubt was, how client should handle receiving an out of rage code
(Eg 600).    Now it is clear, client should gently drop the connection
with QUIT. 
Thank you,
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