Apex domains cannot have CNAME records.
Question and response
Question
dig mailcow.de. NS
Answer
mailcow.de. 5 NS dexter.ns.servercow.net.
mailcow.de. 5 NS angus.ns.servercow.de.
mailcow.de. 5 NS galloway.ns.servercow.com.
Authority
The response has no authority records.
Additional
angus.ns.servercow.de. 5 A 5.1.76.153
angus.ns.servercow.de. 5 AAAA 2a00:f820:417:153:0:0:0:0
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0