As administrator would break a sweat at the mention of malware. Malware are intrusive software that install on a computer without the user's consent. And obviously they don't do so for a productive reason. But administrators rejoice. Blocking malware is pretty simple irrespective of the MTA you are using.
The Malware Block List project does exactly what it sounds like. It maintains a list of URLs known to be infiltrated with viruses, worms, trojans, and the works. Since different MTA use different formats to keep track to blacklists, the malware list is available in several formats. You'll find lists that you can use with Postfix, Squid, SpamAssassin, MailWasher, etc.
Using the list with your MTA is fairly easy. All you need to do is pull in the list manually or automatically via a cron job, and edit your MTA's configuration file to check against this block list. If there's a match, mark it as malware and reat it as you want to -- delete it outright or mark it as spam. To further help admins, the project also hosts HOWTOs for using the blocklist on popular MTAs like Postfix, Squid, and the others mentioned earlier.