We are frequently worried because our e-mail account has received numerous "mail rejected" notifications for messages that we never sent. One PC Magazine editor got over 300 such messages in one night! You may be worried that this spate of rejections is caused by a virus. The good news is that this symptom generally doesn't indicate that you've got a virus. The bad news is that there's no way to block the flood of rejection messages.

Here's what happens. Someone who has your e-mail address gets hit with an e-mail–based virus. The virus harvests all the e-mail addresses it can find from the victim's address book and possibly from e-mail messages and other documents. It then secretly sends itself to all these addresses, attempting to spread the infection further.

Usually some of the harvested addresses will be invalid and hence will cause "mail rejected" messages. After a flood of such messages, the victim might be inspired to investigate or run a full virus scan. To protect itself from this eventuality, the virus "spoofs" the return address on the mail it sends out, using an address chosen randomly from its collection. The rejection messages go elsewhere, and the victim remains blissfully unaware.

Unless you can determine just who among your contacts is the actual victim, there's nothing you can do to prevent the rejection messages. Nor can you automatically discard all rejection messages, since some of them may result from one of your own correspondents legitimately changing to a new e-mail address. But look on the bright side: At least your system isn't infected.

January 2006