Counterattacking the email spammers
According to this Wall Street Journal story (subscription required):
The most-common spam defense used to date -- software filters that attempt to identify and block out the unwanted messages -- hasn't stopped the flood of Viagra pitches, cut-rate mortgage offers, and solicitations for foolproof investment schemes swamping many inboxes. Some recent studies say 50% to 75% of e-mails carried over the Internet are spam.
But now the practice [of counterattacking the spammer with return emails] is going mainstream. International Business Machines Corp. is expected to unveil today its first major foray into the anti-spam market with a service, based on a new IBM technology called FairUCE, that uses a giant database to identify computers that are sending spam. One key feature: E-mails coming from a computer on the spam list are sent directly back to the machine, not just the e-mail account, that sent them. The more spam that comes out, the more vigorous the response.