Apex domains cannot have CNAME records.
Question and response
Question
dig mymail.eu.com. NS
Answer
mymail.eu.com. 5 NS ns-189-b.gandi.net.
mymail.eu.com. 5 NS ns-211-a.gandi.net.
mymail.eu.com. 5 NS ns-231-c.gandi.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0