On Mon, Apr 17, 2023 at 08:54:37AM +0100, Graeme Fowler via Exim-users wrote:
> > How might I configure my routers to ignore an initial 5xx response from the
> > first router and attempt another (and maybe future) deliveries through an
> > alternate router?
>
> If you get a 5xx error from the receiver's MX, you do the right thing and
> abide by it. They're telling you they didn't want your message.
A mail transaction (transmission of a particular message), begins at the
MAIL command and ends at DOT. Any errors outside that context are not
message specific. In particular, they might simply reflect the
unwillingness of the host in question to accept any mail, which may not
be the case with other MX hosts.
In decades past, when now popular MTAs (Exim and Postfix) were new and
evolving their basic SMTP protocol features, it was not uncommon for for
some (IIRC Microsoft Exchange) receiving systems to intermittently
return 5XX when their load was too high.
Consequently, at least Postfix was then, and is still by default now
"tolerant" of 5XX greetings:
smtp_skip_5xx_greeting = yes
This is limited to just the initial banner, not EHLO or later, so
apparently transient misguided 5XX responses to EHLO are not a common
problem. Therefore, I'd be inclined to consider also 5XX in response
to EHLO as a reason to abandom delivery and bounce the envelope.
--
Viktor.
