bmail.btactic.com has no CNAME records.
Question and response
Question
dig bmail.btactic.com. NS
Answer
The response has no answer records.
Authority
btactic.com. 5481 SOA dns16.ovh.net. tech.ovh.net. 2025032500 86400 3600 3600000 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0