Here's what happens: you'll get a "returned mail" from what looks like your own postmaster, but it's not from your postmaster, and you never sent the mail. It will usually have an attachment that says, "the attachment is the original mail." Well, if you open that attachment, guess what? Then you've got a virus.

I kept getting these at my usa.net address, and they told me the mails were not from them, that they were a malicious attempt to proliferate a virus and should be deleted.