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!