I believe the new Spamassassin v3 has addded the ability to add RBL's (remote black lists) that block the entry of mail from known spam sources into the e-mail system in the first place. Therefore, it will never be forwarded anywhere. The RBL's also have the added bonus of reducing the overhead of scan and sorting e-mail you don't really want in the first place. I highly recommend Jesper's work, he's a gem.