Apex domains cannot have CNAME records.
Question and response
Question
dig centron-mailhost.de. NS
Answer
centron-mailhost.de. 5 NS ns2.internet1.de.
centron-mailhost.de. 5 NS ns.internet1.de.
centron-mailhost.de. 5 NS ns3.internet1.de.
Authority
The response has no authority records.
Additional
ns.internet1.de. 5 A 188.64.59.200
ns2.internet1.de. 5 A 62.146.37.200
ns3.internet1.de. 5 A 158.181.50.250
ns.internet1.de. 5 AAAA 2a00:6140:103:0:0:0:0:200
ns2.internet1.de. 5 AAAA 2a01:138:a03e:0:0:0:0:200
ns3.internet1.de. 5 AAAA 2a00:6140:117:0:0:0:0:200
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0