smtp.cacth.net.tw has no CNAME records.
Question and response
Question
dig smtp.cacth.net.tw. NS
Answer
The response has no answer records.
Authority
net.tw. 900 SOA a.twnic.net.tw. snw.twnic.net.tw. 1999210963 3600 900 604800 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0