smtp.netcup.net has no CNAME records.
Question and response
Question
dig smtp.netcup.net. NS
Answer
The response has no answer records.
Authority
netcup.net. 1439 SOA acns01.xaas.systems. mail.netcup.de. 3583 14400 1800 604800 7200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0