|One of the big problems that both junk mail senders and spammers share is lots of dead addresses in their lists. While it doesn't cost anything for a spammer to send an e-mail to a dead address, there is an advantage to knowing an e-mail address is good. Most spammers make money by reselling their lists. If they can increase their "live person" list, it makes the list more valuable to advertisers.
So how does a spammer clean up the list? By getting the recipients to respond to their spam e-mail. The worst thing you can do is actually buy something from a spammer. Even if you don't get taken by a snake oil sex life enhancers scheme, a purchase puts you on the "a REAL live one" list. For most recipients, spam is an annoyance, which they'd like to do something about. Spammers know this, so they put a message at the bottom of the spam saying "click here to unsubscribe." The unsuspecting user then clicks and enters their information and hopes never to see spam again. The result, though, is usually that the user gets more spam sent to their now "valid" e-mail address.