Apex domains cannot have CNAME records.
Question and response
Question
dig post.th.com. NS
Answer
post.th.com. 1 NS ns2.ksc.co.th.
post.th.com. 1 NS ns.ksc.co.th.
Authority
The response has no authority records.
Additional
ns.ksc.co.th. 1 A 203.155.33.1
ns2.ksc.co.th. 1 A 202.44.144.33
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0