So you run some mail server?
sendmail, qmail, whatever it is *nixish based and you don't worry about that much your *nixish system maybe affected (how could any of those nasty windings virii or worm ever anyway ? )but you DO care about those nasty worms and virii transferred via your server?
Well try ClamAV , nope that amn't no advert and i don't get even a cent from them posting this here..it's just outta concern to use some real selftested piece of open source softwarethat does the job and that does it really really good . It's ported already to most *nixish OS's (the few lackin on the list are yet werk in progress..., ya dun happen to run qmail on qnx do ya ? ;-) )..do try it, do use it, i can only strongly recommend it, make sure ya read all the docs and so on, nuff said here is the link : ClamAV