Apex domains cannot have CNAME records.
Question and response
Question
dig flexmail.net. NS
Answer
flexmail.net. 3 NS ns1.combell.eu.
flexmail.net. 3 NS ns3.combell.net.
flexmail.net. 3 NS ns4.combell.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0