Updated: January 21, 2021
What Is DNSBL?
A DNSBL (DNS-based Blackhole List, Block List, or Blacklist) is a list of IP addresses published through the Internet Domain Name Service in a particular format. A blacklist usually refers to a list of email or IP addresses known to send spam emails or some other type of unsolicited messages.
Such lists are currently used by mail servers for filtering incoming emails and blocking the ones listed, in order to improve mail security and integrity. The blacklist is also the opposite of what is called a whitelist. Excluding IP or IP ranges from DNSBL checks is also available in Axigen.
One advantage of using DNSBLs in your AntiSpam strategy is the fact that it rejects connections at TCP level, thus being a great first layer of protection and not creating unwanted load on your inner layers, such as the content filtering engine, which generally require significantly more computing power and / or bandwidth.
One thing to note is that a connection is not rejected if the sender is authenticated even if the connecting IP is listed in one of the DNSBL operators used.
Check out our other page for more information on setting up a DNS blackhole list (DNSBL)