mx2e7b.netcup.net has no CNAME records.
Question and response
Question
dig mx2e7b.netcup.net. NS
Answer
The response has no answer records.
Authority
netcup.net. 6713 SOA acns01.xaas.systems. mail.netcup.de. 3578 14400 1800 604800 7200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0