Apex domains cannot have CNAME records.
Question and response
Question
dig mail.go.th. NS
Answer
mail.go.th. 5 NS amber.ns.cloudflare.com.
mail.go.th. 5 NS jobs.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0