A misconfiguration in one of the SpamAssassin rules resulted in some incoming email to be incorrectly flagged as spam. This resulted in those messages to be denied delivery.
The rule in question checks for “Day Old Bread”: newly registered domain names. The bad guys often register new domains for the sole purpose of sending spam. The DOB rule checks the sender address and email contents for recently registered domains, and then increases the spam score (a measure of the probability that a message is spam) a little bit. The problem was that our configuration increased the spam score by too much (a simple typo in the SpamAssassin configuration). Instead of seeing some messages as having some traits of spam, those messages were classified a definite spam. This was not the intention at all.
On the bright side, the assigned spam score was so high that our servers would have completely denied message delivery. In other words, the sender would have received an error message and the problem would have been quite obvious.
We have corrected the configuration and all is back to normal again. If your senders complained about bounced messages, please ask them to re-send. We regret any inconvenience caused.