Apex domains cannot have CNAME records.
Question and response
Question
dig mailing.diademys.com. NS
Answer
mailing.diademys.com. 5 NS ns1.diademys.com.
mailing.diademys.com. 5 NS ns2.diademys.fr.
Authority
The response has no authority records.
Additional
ns1.diademys.com. 5 A 46.18.130.10
ns2.diademys.fr. 5 A 46.18.131.74
ns1.diademys.com. 5 AAAA 2a01:4580:0:3:0:0:a:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0