(for use with Sendmail)

With you can combine the services of and

How to use

To use you must setup an MTA, such as Sendmail, to perform a DNS query against

In your Sendmail .mc file add
and create a new configuration file. Then copy to the fixed location for your version of Sendmail. Finally, restart Sendmail.

If you use this configuration do not use the individual configurations for and

Now once an IP connects to your server, the IP is reversed (eg. becomes, prepended to "" and a DNS query is perfomed using this hostname. If the IP address is registered in then the DNS query will yield a reply of, and likewise, for, upon which Sendmail will block the connection. Otherwise there will be simply no IP returned from the DNS query and the connection will be allowed to the SMTP server.

Confirming things are working

After some time has elapsed have a look in your maillog. Run the following command:
$ grep /var/log/maillog
If you get one or more records similar to the following things are working fine. If a mail is not rejected by the RBL then there will be no corresponding entry in the maillog.
Nov 16 22:10:54 mail sendmail[55512]: ruleset=check_relay,, arg2=, [], reject=550 5.7.1 Rejected: listed at