Linux/OSS : SpamAssassin
One of the problems currently plaguing the Internet is that of viruses. A side effect of this is that a huge anti-virus industry has sprung up, offering products which defend against malicious software. Since many viruses are these days transmitted by e-mail, a lot of anti-virus software operates on e-mail systems. Unfortunately, a lot of it is written exceedingly badly, and (amongst other things) sends 'virus warnings' to the purported senders of virus-infected e-mail. Unfortunately, however, almost all recent e-mail viruses fake the sender address (often picking a random one from files found on the virus victim's computer), meaning that such 'virus warnings' end up adding to the problem rather than helping, since they end up going to innocent, unrelated third parties.
To help counteract this problem, I've produced a set of SpamAssassin rules which catch a large amount of these bogus 'virus warnings'. Just drop the ruleset into your SpamAssassin rules directory (typically /etc/mail/spamassassin on a Linux or similar system). You can download the rules below:
NOTE: You are welcome to auto-update this ruleset. However, since most common virus warnings are now caught, the ruleset is changing relatively infrequently (not more than once per week most of the time), so please do not auto-update it more than once a day, and use HTTP conditional gets (check Last-Modified from HTTP HEAD, or use If-Modified-Since or If-Not-Match) if at all possible to avoid re-downloading a version that you've already got. I recommend using RulesDuJour which will do the above plus make managing updates of these and other rules easy. RDJ now includes the necessary settings for this ruleset by default.