[exim] Problem with attempted deliveries to A-records instea…

I'm having a weird, intermittent problem.

I run a mailing list server, and most of the time, delivery is fine.
But every so often, Exim decides to try delivering to the A-records of a
domain instead of the MX. It will do this just moments after and prior
to successful deliveries to that same address.

It's doing this for lots of domains, and multiple days in a row for the
same ones, so I find it unlikely that it's a DNS problem outside of my
server. I half suspect a problem with my own DNS server, though it's a
pretty stock BIND8 installation. Possibly caused by load problems,
since the mailing list server dumps a lot of messages at once, but the
load is never all that high, nor is the network traffic. We switched to
using -odqs when submitting messages and the problem has not occurred in
the last fifteen hours, though that's not a clear indicator, as the mail
traffic's been light.

Thing is, if DNS is failing, it should be returning a failure, which
shouldn't cause Exim to try the A-records, right? Exim should only try
A-records when it gets a definitive "no MX exists" response.

It's frustrating to troubleshoot because it's intermittent... if I
"exim4 -d+route -bt fredfish@???" at any given point, it tells me
it's going to route it to the MX.

Any clues as to what I should look at and for?

Thanks!

--
Carl D Cravens (raven@???)
Error locating MAFIA.EXE - program not executed.