A published list of IP addresses that can be queried through the Internet. DNSBLs are used to publish IP addresses associated with e-mail spam and spamming. Most mail servers can be configured to reject messages from addresses on a DNSBL. An address found in a DNSBL may be directly associated with spam, or may have made the list due to Web server vulnerabilities that can be used by spammers. There are many DNSBLs available, each published and maintained by different individuals and organizations.

See also Realtime Blackhole List.

Blacklists, Blocklists, DNSBL's, and Survival
How to survive as a non-combatant emailer in the Spam Wars. A collection of frequently asked and too-often poorly answered questions.

Open Relay Database
A non-profit organization which stores IP addresses of verified open SMTP relays.

SORBS - Fighting spam by finding and listing exploitable servers.
The Spam and Open Relay Blocking System (SORBS) was conceived as an anti-spam project where a daemon would check "on-the-fly", all servers from which it received email to determine if that email was sent via various types of proxy and open-relay servers.