mx.tfn.net.tw has no CNAME records.
Question and response
Question
dig mx.tfn.net.tw. NS
Answer
The response has no answer records.
Authority
tfn.net.tw. 7200 SOA dns1.tfn.net.tw. dns_admin.tfn.net.tw. 2025090406 10800 300 604800 7200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0