mta6.expertsender.com has no CNAME records.
Question and response
Question
dig mta6.expertsender.com. NS
Answer
The response has no answer records.
Authority
expertsender.com. 3600 SOA pdns07.domaincontrol.com. dns.jomax.net. 2026020200 28800 7200 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0