+ https://0spam.org/
+ clear information on usage policy
+ Nothing. The 0Spam Project is absolutely free for email providers, IT professionals and general removal request.
+ DNSBL service and removals are 100% for free for all uses; commercial, non profit and personal.
+ seems serious
+ listing policies are a little less clear
+ bl.0spam.org DNSBL | 0spam Spam Trap Primary Database
+ nbl.0spam.org Network Black List | Spam Source Networks, high volume of spam trap hits in a Class C block will result in network listings in this DNSBL.
+ url.0spam.org URL Black List | This list contains the IP address of domains found to be in the source of spam emails found in our traps.
+ return codes not very clear
+ possibly inactive?
+ https://www.blalert.com/dnsbl/0spam.fusionzero.com