What is a DNS Blacklist (DNSBL)?
A type of filter based on the internet address of an incoming email connection. Basically, mail arrives with a “hello” from another email server. The DNSBL filter is built to lookup and compare the incoming IP address to an online listing of undesirable IP addresses. This listing resides as a “zone” or a special domain on a DNS server. If the incoming address is blacklisted, (i.e. if a match on the IP address is found) the connection is rejected. This process happens early in a SMTP (Simple Mail Transport Protocol) transaction.
