I get hundreds of spams per day, a large percentage of which are "virus spams," the Microsoft worms/viruses that try to infect my Mac. And a significant number of those are bounce messages telling me that the mail that "I sent" contained a virus. Except, of course, that I never sent that mail.
Can someone clue these people in, the ones that run these "helpful" systems, that it is likely that none of the virus spams they are getting actually come from the user in the From: address? They are contributing to the problem. They are not helping.