Here is some updated information from them:

RFC² is not a list of spammers.

RFC Ignorant

With RFC Ignorant closed down, we've expanded our initial RBL to include their lists. You can check to see if a hostname is contained in our lists by attempting to resolve one of the following patterns (generally you need only check whether the host resolves or not rather than worry about the exact IP).

• hostname.dsn.rfc-clueless.org resolves to 127.0.0.2
• hostname.postmaster.rfc-clueless.org resolves to 127.0.0.3
• hostname.abuse.rfc-clueless.org resolves to 127.0.0.4
• hostname.whois.rfc-clueless.org resolves to 127.0.0.5 (or 127.0.0.7 if the TLD is listed)
• hostname.bogusmx.rfc-clueless.org resolves to 127.0.0.8

Meta-lists

You can query all lists at once by querying hostname.fulldom.rfc-clueless.org, which will resolve to 127.0.0.1 if hostname is listed in any of our lists. We have also the std meta-list (dsn, postmaster, abuse, and bogusmx) which uses 127.0.0.1 for listed hosts as well.

For more information, we direct you to the list of frequently answered questions