mx.decknet.net has no CNAME records.
Question and response
Question
dig mx.decknet.net. NS
Answer
The response has no answer records.
Authority
decknet.net. 3436 SOA ns1.decknet.com. postmaster.decknet.fr. 2021072601 10800 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0