How a DNS blocklist lookup works
The address is reversed and placed in front of the list's zone: 192.0.2.1 becomes 1.2.0.192.zen.spamhaus.org. No answer means not listed. An answer in 127.0.0.0/8 means listed, and the last number says why. A TXT record often holds a link with details.
For mail server operators
If your outgoing mail server is listed, fix the cause first (an open relay, a compromised account, a script sending spam), then request removal on the list's site. Make sure the server's address has reverse DNS that matches its hostname; many receivers reject mail without it.
Questions
- Why are some lists IPv4 only?
- Several blocklists never added IPv6. Spamhaus ZEN and DroneBL support both.