I don't know exim, but the postfix site (www.postfix.org
) has plenty of howtos from the MTA to pop/imap servers, to web mail and virus/spam filtering. I'm sure the other MTA's have similar info. Personally I use postfix (if you hadn't guessed) after hearing that it was easier to configure than sendmail (which I think it is).
Also, perhaps have a look at http://www.ijs.si/software/amavisd/
, for amavisd-new, a content filter that interfaces with at least sendmail, postfix and exim to link virus (eg clamav) and spam (eg spamassassin) filters to the MTA.