In black and white: how to use anti-spam lists
Most mail servers or spam filters integrated into mail servers are configured to compare IP addresses with DNS blacklists and, should they score a hit, to decline email from that IP address, or to mark it as potential spam. As an alternative to scoring using multiple criteria, whitelists such as dnswl.org are also now being used. Technically, DNSWLs work in exactly the same way as DNSBLs, except that possible hits are assessed in the opposite manner.
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