Apex domains cannot have CNAME records.
Question and response
Question
dig mailcloud.com.tw. NS
Answer
mailcloud.com.tw. 4 NS ns-444.awsdns-55.com.
mailcloud.com.tw. 4 NS ns-1873.awsdns-42.co.uk.
mailcloud.com.tw. 4 NS ns-1166.awsdns-17.org.
mailcloud.com.tw. 4 NS ns-848.awsdns-42.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0