Software-Gate   

Spam refers to the use of electronic mail medium to send unsolicited messages. E-mail spam is a subset of spam that involves sending nearly identical messages to numerous recipients by e-mail.

Antispam software that lets you get back at spammers that choose to spam your e-mail. Better than a regular email anti spam filter (i.e., for Outlook Express, etc.) because you can actually get back at the spammers. And better still, this is freeware!