mta-staging.expertsender.com has no CNAME records.
Question and response
Question
dig mta-staging.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