Apex domains cannot have CNAME records.
Question and response
Question
dig servicemail24.de. NS
Answer
servicemail24.de. 5 NS ns1.arvato-systems.de.
servicemail24.de. 5 NS ns2.arvato-systems.de.
servicemail24.de. 5 NS dns009.arvato-systems.de.
servicemail24.de. 5 NS dns017.arvato-systems.de.
Authority
The response has no authority records.
Additional
ns1.arvato-systems.de. 5 A 84.17.168.69
ns2.arvato-systems.de. 5 A 109.235.143.148
dns009.arvato-systems.de. 5 A 193.189.249.157
dns017.arvato-systems.de. 5 A 84.17.190.100
ns1.arvato-systems.de. 5 AAAA 2a01:4a8:3222:2222:0:0:0:10
ns2.arvato-systems.de. 5 AAAA 2a01:4a8:2222:2222:0:0:0:11
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0