How do I set up a Qmail/Spamassassin Relay
 

I am trying to find documentation on Qmail relating to a specific usage, but haven't had much luck. I am trying to set up a qmail relay which will take incoming mail and send it through spamassassin, then forward the mail on to our Exchange server if the mail isn't spam. I don't want to have to create user accounts on the box running qmail, I simply want the box to remove spam from incoming mail, then send it on it's way. Has anyone done this before, or does anyone know where I can find a HOWTO on this?
Thanks!