Apex domains cannot have CNAME records.
Question and response
Question
dig mailbox.org. NS
Answer
mailbox.org. 5 NS ns.jpberlin.de.
mailbox.org. 5 NS ns3.jpberlin.de.
mailbox.org. 5 NS dns3.cloudns.net.
mailbox.org. 5 NS dns4.cloudns.net.
mailbox.org. 5 NS ns2.jpberlin.de.
Authority
The response has no authority records.
Additional
ns.jpberlin.de. 5 A 91.198.250.3
ns2.jpberlin.de. 5 A 194.150.191.56
ns3.jpberlin.de. 5 A 88.198.52.15
ns.jpberlin.de. 5 AAAA 2001:67c:2050:1:0:0:53:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0