mx.flatmail.com has no CNAME records.
Question and response
Question
dig mx.flatmail.com. NS
Answer
The response has no answer records.
Authority
flatmail.com. 3554 SOA ns11.infomaniak.ch. hostmaster.infomaniak.ch. 2023051748 10800 3600 605800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0