mail.netcup.net has no CNAME records.
Question and response
Question
dig mail.netcup.net. NS
Answer
The response has no answer records.
Authority
netcup.net. 7200 SOA acns01.xaas.systems. mail.netcup.de. 3582 14400 1800 604800 7200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0