listing policies are clearly documented
return codes are clearly documented
http://www.surbl.org/
+ good reputation
+ lists domains/URIs
+ usage policy is clear
+ https://surbl.org/usage-policy
+ listing policy is documented
+ return codes are documented
+ bitmasked
+ 127.0.0.1 means blocked
http://zapbl.net/
https://rbl.foobar.hu/
http://apews.org/?page=filter