This is a thread for REALLY HARD computer questions

I'm debugging some SMTP problems with one of our client's mailservers. They got cracked a while back and were used for sending spam and thus got on a bunch of spam blackhole lists. We can't receive email from them, but it appears our RBLs don't filter them, so I'm wondering if its something else.

I'm running qmail which is pretty nasty about enforcing its own rules on people. How should I go about figuring out the problem?? :(((