Also, use the various RBLs in the scoring. e.g. add 50% of the threshold
score if its on spamcop and 25% for some of the other more aggressive
RBLs. We have a very high and correct hit rate as a result. Our users can
then add white lists for the handful of their contacts that get tagged as
spam since they are using spam friendly ISPs.
Not to thread jack or anything, but when I first moved our cluster to
Spam Assassin, I was disappointed at the amount of messages that would
get past Spam Assassin at even a low threshold of 2.
I Googled around and found a bunch of rulesets that once installed,
started tagging those hard to get messages.