mail.tgtnet.com has no CNAME records.
Question and response
Question
dig mail.tgtnet.com. NS
Answer
The response has no answer records.
Authority
tgtnet.com. 7778 SOA hkdc1-ccns3.hdcdatacentre.com. dns_admin.tgt.hk. 2023020801 10800 3600 604800 38400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0