Apex domains cannot have CNAME records.
Question and response
Question
dig sul.t-online.de. NS
Answer
sul.t-online.de. 5 NS dns00.sul.t-online.de.
sul.t-online.de. 5 NS dns00.sda.t-online.de.
sul.t-online.de. 5 NS dns01.sul.t-online.de.
sul.t-online.de. 5 NS dns01.sda.t-online.de.
Authority
The response has no authority records.
Additional
dns00.sul.t-online.de. 5 A 62.153.158.62
dns01.sul.t-online.de. 5 A 194.25.134.203
dns00.sul.t-online.de. 5 AAAA 2003:2:2:140:fee:0:0:62
dns01.sul.t-online.de. 5 AAAA 2003:2:2:140:fee:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0