Apex domains cannot have CNAME records.
Question and response
Question
dig mail2000.com.tw. NS
Answer
mail2000.com.tw. 5 NS ns-1531.awsdns-63.org.
mail2000.com.tw. 5 NS ns-1848.awsdns-39.co.uk.
mail2000.com.tw. 5 NS ns-183.awsdns-22.com.
mail2000.com.tw. 5 NS ns-719.awsdns-25.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0