https://knowledge.validity.com/hc/en-us/sections/204468388-Blocklists
https://github.com/zbetcheckin/DNSBLs
https://en.wikipedia.org/wiki/Comparison_of_DNS_blacklists
https://knowledge.validity.com/hc/en-us/sections/204468388-Blocklists
https://github.com/zbetcheckin/DNSBLs
https://en.wikipedia.org/wiki/Comparison_of_DNS_blacklists
- http://spamcannibal.org/dnsbl_check.shtml
+ usage policy not explicit but seems to be free
+ listing policy also not explicit but can be inferred from return code explanation
+ return codes are explained