mail.cntcloud.com has no CNAME records.
Question and response
Question
dig mail.cntcloud.com. NS
Answer
The response has no answer records.
Authority
cntcloud.com. 2906 SOA ns21.domaincontrol.com. dns.jomax.net. 2026020502 28800 7200 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0