mail1dev.testback.com has no NS records.
NS records for this zone are defined at testback.com.
NS records for this zone are defined at testback.com.
Question and response
Question
dig mail1dev.testback.com. NS
Answer
The response has no answer records.
Authority
testback.com. 3600 SOA ns1.dnshost.net. zonemaster\@dnshost.net. 1749028927 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1400, xrcode 0, version 0, flags 0