mail.cdnsun.com has no CNAME records.
Question and response
Question
dig mail.cdnsun.com. NS
Answer
The response has no answer records.
Authority
cdnsun.com. 181 SOA gns1.cloudns.net. dns.cdnsun.com. 2025120501 43200 7200 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0