We tromped back to the office and banged on it for about an hour before we managed to reproduce the symptom, then worked on the fix until 4 a.m. I finally left the office at 4:30, and got home just before 5 a.m. Long day, that.
I just sorted out about a week's worth of mail on my backgammon server. Of 945 messages:
106 were spam
807 were worms/viruses
15 were warnings about worms/viruses (but I deletd a couple hundred earlier from a different folder.)
leaving a whopping 17 that were actual mail messages. Of those, about half were test messages that I sent from work when playing with a sendmail config. I finally null-routed about half a dozen hosts.