Apex domains cannot have CNAME records.
Question and response
Question
dig d39s-mail.de. NS
Answer
d39s-mail.de. 1 NS rustam.ns.cloudflare.com.
d39s-mail.de. 1 NS adel.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0