I think there's a flaw in the Spam filtering.
I had several mails delivered today that weren't tagged, even though they were from blacklisted addresses. The Spam filtering software had even added the header that said "listed in a blacklist at Spamcop" but still didn't tag it as Spam. Surely if an address is blacklisted that is enough to class it as Spam without any further factors?
I am religious about training the filters - I send all untagged Spams to despamchecker, but sometimes I wonder if I am wasting my time.