This internally used RBL is maintained by Schlund+Partner AG and 1&1 Internet AG to discover relay servers and block them.
The list is generated by active relay checks. When emails are received by this group the sender host is scheduled to be checked for being an open relay. Relaytest.kundenserver.de attempts to relay a mail via this host and as soon as the mail is received at relaytest.kundenserver.de, the IP is added to the list as an open relay.
Is the information on our site wrong or incomplete? You don't want us to monitor your blacklist? Or do you operate a similar blacklist? We would like to hear from you!
We keep some statistics on blacklists. Below is the graph for relays.bl.kundenserver.de. We test blacklists by asking them to respond to known IP's that are blacklisted and known IP's that can't be blacklisted. Not every blacklist has such IP's, we test them with what is possible. We keep if we get any response or error messages and the response duration. Lower response times are better. These statistics may be useful for email server administrators to pick which blacklist to use.