Mac OS X Mail can help you deliver a staggering blow to spammers. Simply pull down the Mail menu, choose Junk Mail, and select Automatic. The next time you receive email, Mail will move suspect email into a Junk folder.
With that done, you’re ready to deliver a real knockout punch to spammers by taking advantage of yet another potent spam-fighting weapon:
1. Click on the Junk folder.
2. Type Command-a to select all of the Junk email messages.
3. Choose “Bounce to Sender” from Mail’s Message Menu.
Mail will return the selected messages to the senders marked ‘User unknown,’ making them think your email address invalid, encouraging them to drop you from their lists, and, thus, eliminating spam at its source.
(From Apple eNews, 6 Feb 2003)