Apex domains cannot have CNAME records.
Question and response
Question
dig mailus.de. NS
Answer
mailus.de. 5 NS a.ns.mailus.de.
mailus.de. 5 NS b.ns.mailus.de.
mailus.de. 5 NS c.ns.mailus.de.
Authority
The response has no authority records.
Additional
a.ns.mailus.de. 5 A 178.21.144.21
b.ns.mailus.de. 5 A 178.21.145.21
c.ns.mailus.de. 5 A 178.21.146.21
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0