Been running SpamAssassin on the mail server for quite some time.
The following .cf files have reduced the amount of spam by a noticeable bit.
Throw them in your
/etc/mail/spamassassin/ directory and restart spamd. Oh yeah, make sure and :
mv sa-blacklist.current 50blacklist.cf.
Make sure you keep them up to date, that way you will be blocking all the new stuff..