mail.dnsresolve.net has no CNAME records.
Question and response
Question
dig mail.dnsresolve.net. NS
Answer
The response has no answer records.
Authority
dnsresolve.net. 1237 SOA ns1.dnsresolve.net. root.ns1.dnsresolve.net. 2004093004 40000 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0