Today I learned how to query if an address is on an rbl list.
Reverse the octets of the IP address:
Then append the domain name given by the DNSBL service (
Then look up of this domain's A record.
If the IP is listed, it will return an address, or it will return an NXDOMAIN if the IP is not listed. The exact meaning of the address can be different at each provider.
Based on the knowledge I created a small program to check if a address is listed on a blacklist: DNSBL Check