Backscatter - It's Okay to Be Impolite
In the world of unsolicited commercial email (a.k.a. spam), the perpetrators who send these messages often count on you being polite, or at least they count on your mail server being polite. Back in 1982 when the whole idea of email was first taking form, the Internet Engineering Task Force set some standards. One of the requirements for a properly-functioning mail server was this: if you receive a message to an invalid address, you should politely return the message telling the sender that you could not deliver it.