mx1.workd.go.th has no CNAME records.
Question and response
Question
dig mx1.workd.go.th. NS
Answer
The response has no answer records.
Authority
workd.go.th. 1740 SOA amber.ns.cloudflare.com. dns.cloudflare.com. 2397905709 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0