Apex domains cannot have CNAME records.
Question and response
Question
dig tutanota.de. NS
Answer
tutanota.de. 5 NS ns-1344.awsdns-40.org.
tutanota.de. 5 NS ns-1757.awsdns-27.co.uk.
tutanota.de. 5 NS ns-478.awsdns-59.com.
tutanota.de. 5 NS ns-998.awsdns-60.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0