mxb.bak.secmail.com has no CNAME records.
Question and response
Question
dig mxb.bak.secmail.com. NS
Answer
The response has no answer records.
Authority
secmail.com. 1952 SOA dns1.solutions.fi. dnsmaster.solutions.fi. 2026022104 10800 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0