Hi,
Many thanks to Philip for his comprehensive response.
In response to another post, we are running Exim 3.36 under Debian
'testing' distribution. 'unrouteable mail domain' errors occur when
*sending* messages.
> > We've been experiencing a problem whereby MX records fail to be found for
> > some domains when exim does a DNS lookup (NB as distinct from timing out
> > when trying to do the MX lookup). In this case, exim appears to
> > bounce the message immediately.
>
> Well, that's the correct behaviour. The domain is unknown.
>
> > This appears to be an intermittent
> > problem, as the MX lookup is found OK with emails at other times.
>
> The usual reason for this kind of behaviour is that one of the DNS
> servers for the zone has got out of step with the others. You can test
> this by finding the name servers for the zone, and looking up the problem
> domain in each of them explicitly. If the MX records are present in some
> name servers and not in some others, that explains why you sometimes
> find them and sometimes not.
We haven't found any evidence of this yet, but will put a script in place
to examine the state of MX records for domains as soon as each
'unrouteable mail domain' error occurs.
> > 2. Can exim be made to retry in the event of no MX records being found?
>
> No.
Unfortunately, given that this problem appears to occur intermittently,
the fact that Exim cannot be made to retry is a problem for us, as
messages keep bouncing back and have to be resent. We deliver information
to customers via email, and if Exim cannot be made to retry, we'll have to
try to put systems in place to pick up the bounces and re-send the
message. It would be easier for us if Exim could be reconfigured to get
the desired behaviour.
I'm going to experiment with modifying lookuphost.c to defer the message
if no MX or A records are found, and will let you know what transpires.
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Tel: +44 (0)20 7471 0400
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