Apex domains cannot have CNAME records.
Question and response
Question
dig sps-mm.de. NS
Answer
sps-mm.de. 5 NS cns1.alfahosting.info.
sps-mm.de. 5 NS cns3.alfahosting.info.
sps-mm.de. 5 NS cns2.alfahosting.info.
Authority
The response has no authority records.
Additional
cns1.alfahosting.info. 2 A 109.237.142.8
cns2.alfahosting.info. 2 A 148.251.254.105
cns3.alfahosting.info. 2 A 109.237.143.8
cns1.alfahosting.info. 2 AAAA 2a01:64c0:0:5000:0:0:0:8
cns2.alfahosting.info. 2 AAAA 2a01:4f8:2b02:a88:0:0:0:105
cns3.alfahosting.info. 2 AAAA 2a02:2b80:101:5000:0:0:0:8
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0