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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.