mbssmtp.mbsbooks.com has no NS records.
NS records for this zone are defined at mbsbooks.com.
NS records for this zone are defined at mbsbooks.com.
Question and response
Question
dig mbssmtp.mbsbooks.com. NS
Answer
The response has no answer records.
Authority
mbsbooks.com. 1800 SOA chan.ns.cloudflare.com. dns.cloudflare.com. 2399110385 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0