mailtx.inet.co.th has no CNAME records.
Question and response
Question
dig mailtx.inet.co.th. NS
Answer
The response has no answer records.
Authority
inet.co.th. 300 SOA ns2.inet.co.th. root.inet.co.th. 29693 3600 600 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0