Reverse DNS (rDNS) is a method of resolving an IP address into a domain name, just as the domain name system (DNS) resolves domain names into associated IP addresses. One of the applications of reverse DNS is as a spam filter. Here's how it works: Typically, a spammer uses an invalid IP address, one that doesn't match the domain name. A reverse DNS lookup program inputs IP addresses of incoming messages to a DNS database. If no valid name is found to match the IP address, the server blocks that message.
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Although reverse DNS is fairly effective for filtering spam, it also sometimes blocks valid e-mail, at least in the current technological environment. A number of problems, including network delays and improperly configured networks or servers, can prevent legitimate messages from getting through the filter. In January 2003, AT&T WorldNet started using reverse DNS in conjunction with other anti-spam software. The company was forced to remove the filter just 24 hours after it was deployed, after subscribers reported that messages were going undelivered.