Apex domains cannot have CNAME records.
Question and response
Question
dig inbox.eu. NS
Answer
inbox.eu. 4 NS ns3.inbokss.com.
inbox.eu. 4 NS ns1.inbokss.com.
inbox.eu. 4 NS ns2.inbokss.com.
Authority
The response has no authority records.
Additional
ns1.inbokss.com. 4 A 194.152.32.8
ns2.inbokss.com. 4 A 194.19.227.100
ns3.inbokss.com. 4 A 52.149.100.222
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0