What is a DNSBL?
Domain Name System Blacklists, also known as DNSBL's, DNS Blacklists or DNS Block Lists are spam-blocking lists that allow a website administrator and ISPs (Internet Service Providers) to block messages from specific systems that have a history of sending spam.
If the maintainer of a DNS Blacklist has in the past received spam of any kind from a specific IP address, that server would be "blacklisted" and all messages sent from it would be either flagged or rejected from all sites that use that specific blacklist.
History of DNSBL.
The first DNSBL was created in 1997 and was called the RBL (Realtime Block List). Its purpose was to block spam email and to educate Internet service providers (ISPs) and other websites about spam and its related problems. Almost all of today's email servers support at least one DNSBL in order to reduce the amount of junk mail clients using their service receive.
Since 1997 many other DNSBL providers have started up and they all have their own lists and methods of determining what is spam. Some are stricter than others. Some sites only list the IP Address of a short time, on others it’s a permanent listing until you manually ask for removal (some sites charge for this). Some DNSBL even block entire address ranges so you could be blocked on a DNSBL simply because of someone else’s spamming activity and also because the ISP hasn’t taken then necessary security measures to combat spamming from their networks.
The results are that some lists work better than others because they are maintained by services with a greater level of trustworthiness and credibility than competing lists might have.