smtp.isl.net.tw has no CNAME records.
Question and response
Question
dig smtp.isl.net.tw. NS
Answer
The response has no answer records.
Authority
isl.net.tw. 5785 SOA dns.isl.net.tw. admin.isl.net.tw. 2025070801 604800 86400 2419200 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0