Fine until it's one of the viruses which reuses bits of old mails for sending itself. This one already occurred in engineering.

You're still OK so long as the infected people haven't mailed you a zip file before, and the virus doens't happen to pick that Subject: line; which is at least better odds.

The thing is the better you do a job of protecting your users from virus-ridden mail the more likely they are to fall for it and get infected if it does somehow get through.