Apex domains cannot have CNAME records.
Question and response
Question
dig webixmail.de. NS
Answer
webixmail.de. 1 NS nikon.ns.cloudflare.com.
webixmail.de. 1 NS mariah.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0