It began in 1997 with only one computer listed. Sites on the list include networks that are friendly or neutral to spam. Another type of site on the list is one that sells spamware - software designed to assist spammers. Open-relays are used by spammers to help cover their tracks. The spammer uses another computer on the Internet, the third-party relay, to send their spam.
Spammers can also use many unsecured relays to send out more e-mail in less time.
This can clog up the relay's servers and can cause denials of service. In some ways, the owners of open relays are also victims, but they are the unwitting tools of spammers. Getting put on the RSS often alerts these sites to the spam problem. Spammers will sometimes try to send mail directly from those ISPs and bypass their provider's mail server. If you block dial-up IPs from sending you mail, you're going to stop that kind of spam from getting to you.