Filter out bad emails

Hello, i use qmail with spamassassin and qmail-scanner. As you all know, most of the spamers generate emailadresse by them mself and try to deliver ther sh** to the "new recepient". The LINUX-Box is only our relay. In the back is a Exchange-Server working. The EX-Server trys to answers to the non existent mailadress of the spammer. Spamassassin has already markted this mails as spam. How can i use MAILDROP (or someting else) to kill this mails. Because this mails make my queue on the qmail-Server grow and grow ... Its not allowed to me to drop all mails marked as spam. But its allowed to drop mails, marked as spam which are send from our own Postmaster. thanks for your Help Robert Rasp