mail.netcup.de has no CNAME records.
Question and response
Question
dig mail.netcup.de. NS
Answer
The response has no answer records.
Authority
netcup.de. 2790 SOA root-dns.netcup.net. dnsadmin.netcup.net. 24 28800 7200 1209600 7200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0