We'd like to protect our customers as well as our own servers from the hazards and risks of open relays as a well-known source for UBE/spam.
That's why we're using internal RBL-style lists for blocking certain hosts.

Since RBL-style lists mean that we'd give out an important configuration option to a third party without any possibility to alter the data or the checks and these changes directly affect many customers, we don't want to rely on third-party services.
That's why we're building our own service.

When our mail cluster receives mail from a host, this host is scheduled to be checked for being an open relay (see http://mail-abuse.org/tsi/ for closer information on the subject of such unsecured mail servers and how to fix this security problem). Relaytest.kundenserver.de attempts to relay a mail via this host and as soon as the mail is received at relaytest.kundenserver.de, we'll list the affected host as an open relay.

Operated by Schlund+Partner AG
and 1&1 Internet AG