dssmtp2.thains.co.th has no CNAME records.
Question and response
Question
dig dssmtp2.thains.co.th. NS
Answer
The response has no answer records.
Authority
thains.co.th. 1800 SOA ns1.thains.co.th. registry.thains.co.th. 2026030301 900 900 864000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0