[Exim] unrouteable mail domain

Hello,

I have noticed a few "unrouteable mail domain" messages appearing in the
exim_mainlog, but in most cases when I look up the domains myself I have
no trouble obtaining an MX record for them.

Is it possible that the DNS lookups are timing out? I thought I had read
somewhere that a DNS timeout should result in the message staying on the
queue, but in these cases the messages are being returned immediately to
the sender.

The only router we have configured is the standard "lookuphost" one.

Am I overlooking something here?

Thanks,

Dave