When you run your own mail server, it's a good idea to make sure the external spam verification systems you use are, well, still around
"At noon today (Eastern Standard Time), the long dead ORDB spam identification system began returning false positives as a way to get sleeping users to remove the ORDB query from their spam filters. The net effect: all mail is blocked on servers still configured to use the ORDB service, which was taken out of commission in December of 2006. So if you're not getting any mail, check your spam filter configuration!"
The wife contacted me because she wasn't getting any mail. Neither was I. Checked the logs and sure enough, ORDB was rejecting everything.
The thing is that not every email goes through those filters. Certain addresses are allowed through regardless, so we were getting some email, so nothing seemed odd until we realized we weren't getting emails we were supposed to be.
Oh well... Live and learn.