Realtime Blackhole List (RBL)

Also known as Blacklist, Domain Name System-based Blacklist (DNSBL) or DNS Blacklist. The term "blackhole list" is also interchanged with the terms "blacklist" and "blocklist".

These lists are maintained to stop email spamming by allowing administrators to block messages from specific locations that have a history of sending spam. If the maintainer of a DNS Blacklist has in the past received spam of any kind from a specific domain name, that server would be "blacklisted" and all messages sent from it would be either flagged or rejected from all sites that use that specific list.

See for more information.