mail1dev.testback.com has no CNAME records.
Question and response
Question
dig mail1dev.testback.com. NS
Answer
The response has no answer records.
Authority
testback.com. 3465 SOA ns1.dnshost.net. zonemaster\@dnshost.net. 1749028927 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0