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List of All Known DNS-based Spam Databases. The most common way of detecting spam is by using spam databases (blacklists, sometimes incorrectly referred to as RBLs, since RBL is trademarked by MAPS) that list the addresses of mail servers known (or believed) to send spam. This is done by taking the IP address of the remote mail server, converting it to a domain name using the ip4r format (1.2.3.4 becomes 4.3.2.1.bl.example.com), and seeing if that name exists.
