mail.netdesign.co.th has no CNAME records.
Question and response
Question
dig mail.netdesign.co.th. NS
Answer
The response has no answer records.
Authority
co.th. 1800 SOA a.thains.co.th. registry.thains.co.th. 1260304010 1800 900 518400 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0