There are many DNS-based blacklists (DNSBLs). These are also known simply as blacklists or blocklists. They provide a service that is used by MTAs and spam filters to indicate sites that are related to spammers. An MTA or spam filter may use one or more blacklists. SpamAssassin can use blacklists to filter spam.
Blacklists can generally be placed in one or more of these categories:
A list of known open relays
A list of known sources of spam
A list of sites hosted by an ISP that encourages spammers in some way
Every blacklist has unique policies for adding and removing domains from the list. Some are very aggressive, and block not only sources of spam, but also any address served by the same Internet service provider. The intention of this approach is to force ISPs to stop doing business with spammers and thus force them off the net. This approach, called the Internet black hole of death has been used with success against major ISPs in the past.
Blacklists provide a spam filter...
