+ http://rbl.schulte.org/
+ seems active
+ listing policy seems to be: they received spam from an IP
+ usage policy: Anyone can use this RBL list [sic]
+ return codes: probably boolean, i.e. either listed or not
+ http://relaytest.kundenserver.de/
+ by 1und1 (now ionos?), used internally
+ https://www.blalert.com/dnsbl/relays.bl.kundenserver.de
+ no usage policy found
+ no listing policy found
+ no return code explanation found