Dnswl.org is the leading email whitelist, also called DNS whitelist (DNSWL).
Dnswl.org provides reputation information about email senders. It has close to 100’000 users (companies, organisations and individuals) with tens of thousands of requests per day, and contains over 300’000 entries of “good mailservers”, including a fast growing amount of IPv6 entries.
dig -t txt amiblocked.dnswl.org ; <<>> DiG 9.9.5-9ubuntu0.3-Ubuntu <<>> -t txt amiblocked.dnswl.org ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 29706 ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0 ;; QUESTION SECTION: ;amiblocked.dnswl.org. IN TXT ;; ANSWER SECTION: amiblocked.dnswl.org. 86400 IN TXT "no" ;; Query time: 422 msec ;; SERVER: 127.0.1.1#53(127.0.1.1) ;; WHEN: Mon Sep 14 21:49:43 CEST 2015 ;; MSG SIZE rcvd: 53
if yes, you should setup yourown caching name server. pdns_recursor?