mailbackup.th.com has no NS records.
NS records for this zone are defined at th.com.
NS records for this zone are defined at th.com.
Question and response
Question
dig mailbackup.th.com. NS
Answer
The response has no answer records.
Authority
th.com. 900 SOA ns1.ksc.net. hostmaster.ns.ksc.co.th. 2021022740 10800 3600 2419200 900
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0