DNSBL
List of known-bad (malicious) email servers on the web
A DNSBL is a list of the IP addresses of known-bad servers, generally spamming email servers. Such lists are made available for spam-blocking software to query in real time, so that unknown addresses requesting connections can be checked before they are granted access.
The technology for operating a DNSBL is fairly simple and there are a variety of such lists provided and maintained by many different organisations, as well as several variations on the same theme. Prominent examples include SpamCop Spamhaus, and others.
