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DNSBL(DNS BlackList or DNS Blackhole List) A system for identifying spammers based on their IP addresses. Most mail servers are able to query a DNSBL to determine if a specific IP address is on the list. The MAPS Real-time Blackhole List (MAPS RBL) was the first DNSBL, but was initially not in the DNSBL format.
There is no uniformity in the way DNSBL providers operate. Candidates for the list may be supplied by users or only from sources that have proven the IP address is from a spammer. Some DNSBL providers list only IP addresses, while others list only open relays and open proxies (see open relay and open proxy server). The policies also differ as to how long a listing lasts and how it can be removed. See MAPS.