This RBL contains all IP addresses and MX hosts that are listed as mailserver for nonexistant domains.
To explain this a little further: Some registries set wildcard records for their top-level domains. An example would be -.tk, which - at the time of writing this text - resolves to four IP addresses and even has an MX record set. If you surf to that domain, you're being told that The domain name you have entered is invalid, yet it works perfectly fine.
This behaviour shows, that the .tk registry set up a wildcard record to link surfers to their own website, who are trying to access an unregistered domain. Unfortunately this means that spammers can just randomly use any .tk-domainname for spamming and pass (some) important spamchecks with it.
The contents of this rbl is being fully automatically reevaluated once a day.
The zone is being served by rbldnsd for maximum performance.