[exim] Slow DNS leads to Unbooteable Addresses

I'm trying to track down the cause of what I think is a two part
problem.

I have a mail server that has been running happily along for several
days. The DNS on it's network is stupidly slow (network admin is
working on that as I write this). Exim seems to give us on lookups very
quickly and return the addresses as unrouteable (since this server runs
mailman I have a list that's getting shorter by the minute).

When I run >sudo /usr/sbin/exim -bt ahc1979@???
I instantly get:
ahc1979@??? is undeliverable:
Unrouteable address

Even though after the lookup on the firewall finally completes I can
ping the address, I still get an unrouteable answer.

Is Exim caching failed DNS entries somehow? Or is this likely to
correct itself when the DNS starts working better? The slow DNS as
intermittently been a problem ever since I setup the network, but exim
seemed to recover most of the time until today. DNS is worse today, but
I'm not aware of any other changes.

Aaron