mail.firstline-engineering.com has no CNAME records.
Question and response
Question
dig mail.firstline-engineering.com. NS
Answer
The response has no answer records.
Authority
firstline-engineering.com. 286 SOA ns2.firstline-engineering.com. postmaster.firstline-engineering.com. 2025112702 10800 3600 1209600 10800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0