Re: [exim] exim can't handle 521 response from remote MX

Hello.

On Thu, Sep 02, 2021 at 09:25:20PM +0200, krzf83--- via Exim-users wrote:
> Then exim should return message to sender immeadetly but it does not.
> Instead exim remembers that that remote mx is "failing for long time" and
> does not even try to deliver new mails! If exim for some reason does retry
> then it logs "Remote host closed connection in response to EHLO" and won't
> retry it again for any emails for that domain! Only after 72 your it
> returns email to sender.

Such behaviour of SMTP server --

> > > # nc mx.poczta.onet.pl 25
> > > 220-mx.poczta.onet.pl ESMTP
> > > 521 5.7.1 Service unavailable; client [144.76.50.172] blocked using
> > > postscreenbl.opbl.onet.pl.local

is a direct violation of SMTP protocol, which has no place for different
status codes in a single reply. This reply may be treated as a temporary
error (as Exim does), for example, as transient data corruption.
Client's reaction is undefined.

Server should NOT begin it's greeting with 220 to make legitime rejection.
Special code 554 is reserved for rejection on greeting - RFC2821, par.3.1:

The SMTP protocol allows a server to formally reject a transaction
while still allowing the initial connection as follows: a 554
response MAY be given in the initial connection opening message
instead of the 220.

Server also must not use extended status codes until client's EHLO.
--
Eugene Berdnikov