Apex domains cannot have CNAME records.
Question and response
Question
dig s01.mail.datingexperte.de. NS
Answer
s01.mail.datingexperte.de. 5 NS dropcatcher.elitedomains.de.
s01.mail.datingexperte.de. 5 NS backorder.elitedomains.de.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0