[AChimp]

Actually, it's just spam that people are sending you with a mailer daemon error in the subject header to trick you.

See, you're supposed to go, "What did I send that was wrong? I better check this out." And then you open it and see a giant cock and become compelled to send all your money to a guy in Cambodia.

It's also simple enough to spoof someone's e-mail address so that it appears in the return path by just generating false header information. Anyone with PHP or ASP skillz can do it in 2 seconds.