I have the same issue! Recently, I noticed that my DNS queries being proxied elsewhere by my ISP (Optimum).
For example, using dnsleaktest, I can see that queries being redirected to:
178.255.84.37 no-dns-yet.ccanet.co.uk CCANET Limited United Kingdom
178.255.84.35 no-dns-yet.ccanet.co.uk CCANET Limited United Kingdom
178.255.84.36 t8sgom.comodogroup.com CCANET Limited United Kingdom
178.255.84.34 no-dns-yet.ccanet.co.uk CCANET Limited United Kingdom
This is not expected and produces less than optimal results for my attempts to check DNSBL's on connections to my mail server - see all 92.242.144.50 responses:
2017-04-23T10:51:47-04:00 nas01 postfix/postscreen[18411]: CONNECT from [71.43.237.170]:49081 to [192.168.2.200]:25
2017-04-23T10:51:48-04:00 nas01 postfix/dnsblog[18956]: addr 71.43.237.170 listed by domain bl.spamcop.net as 92.242.144.50
2017-04-23T10:51:48-04:00 nas01 postfix/dnsblog[19079]: addr 71.43.237.170 listed by domain truncate.gbudb.net as 92.242.144.50
2017-04-23T10:51:48-04:00 nas01 postfix/dnsblog[19081]: addr 71.43.237.170 listed by domain all.usa.bl.blocklist.de as 127.0.0.9
2017-04-23T10:51:48-04:00 nas01 postfix/dnsblog[18960]: addr 71.43.237.170 listed by domain dnsrbl.org as 92.242.144.50
2017-04-23T10:51:48-04:00 nas01 postfix/dnsblog[18959]: addr 71.43.237.170 listed by domain all.rbl.webiron.net as 127.0.0.2
2017-04-23T10:51:48-04:00 nas01 postfix/dnsblog[18955]: addr 71.43.237.170 listed by domain zen.spamhaus.org as 127.0.0.4
2017-04-23T10:51:48-04:00 nas01 postfix/dnsblog[18957]: addr 71.43.237.170 listed by domain bl.blocklist.de as 127.0.0.9
2017-04-23T10:51:48-04:00 nas01 postfix/dnsblog[19079]: addr 71.43.237.170 listed by domain hostkarma.junkemailfilter.com as 92.242.144.50
2017-04-23T10:51:53-04:00 nas01 postfix/postscreen[18411]: DNSBL rank 9 for [71.43.237.170]:49081
2017-04-23T10:51:54-04:00 nas01 postfix/postscreen[18411]: DISCONNECT [71.43.237.170]:49081