What is a Domain DNS Blacklist?

A DNSBL, or a domain name server - based blacklist, DNS block list or domain blacklist is a list of IP addresses referenced by DNS server software to check if an email sender is a known spammer. Most mail servers will check DNSBLs to see if the sender’s domain name is associated with spammers, and occasionally if any of the links contained in the email are spam links. While DNSBLs can be a powerful tool for stemming the flow of spam, there are occasional false positives that result in legitimate email being flagged as spam and rejected by mail servers.