Just clearing something up. The message that he was receiving wasnt a reply from a virus scanner, it was a bounce. I totally agree with you that virus scanners that reply to addresses that are 'sending' viruses are a total waste of time as the sender addresses are always forged.
In this case though, the receiving server is not replying to tell him that he has sent a virus, its telling him that hes sent an email to a nonexistant user. Obviously a message like this can be very useful if you have mistyped an address or something.
As to whether these messages are "junk" or "spam" I guess depends on your definitions and what you expect your spam filter to do. I have no argument that masses of postmaster messages like that are a total pain in the ass, but they are not spam as in the definition of "unsolicited bulk advertising email". Some of them are an annoying byproduct of viruses, but others are there to warn you if there is a problem with delivering your mail and so I think its worthwhile pointing out that there are disadvantages to just blindly filtering them out.
10.0 times 0.1 is hardly ever 1.0.