You probably want to be looking at configuring Spamassassin and/or ClamAV. There's a number of resources on the net about this, such as
http://librenix.com/?inode=4141 (the first result from Google). It's not that hard, and once SpamAssassin is trained a bit, should cut the number of junk mails you get through. It will take up a bit of processing time though for each mail to be scanned + filtered, so you might want to look at your hardware depending on number of users supported + amount of e-mails received daily.
