mailbackup.th.com has no CNAME records.
Question and response
Question
dig mailbackup.th.com. NS
Answer
The response has no answer records.
Authority
th.com. 3236 SOA ns.ksc.co.th. hostmaster.ns.ksc.co.th. 2026021200 3600 1200 129600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0