DNSBL DNS blacklist check tool.
Use this tool to check if given ip address have a history of sending spam. How it works? If your mail server is blacklisted for example by dnsbl.sorbs.net and ip address of your mail server is 184.108.40.206, it means that there is a A DNS record exist in domain dnsbl.sorbs.net for this ip address. This A record uses reverse order of ip address octets and looks like this 220.127.116.11.dnsbl.sorbs.net. If your mail server is blacklisted, you need contact sorbs.net to remove your ip address from blacklist.