Apex domains cannot have CNAME records.
Question and response
Question
dig mail24server.de. NS
Answer
mail24server.de. 5 NS mail24server.com.
mail24server.de. 5 NS internet24server.de.
mail24server.de. 5 NS weltnetz24server.de.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0