mx4.BelWue.de has no CNAME records.
Question and response
Question
dig mx4.BelWue.de. NS
Answer
The response has no answer records.
Authority
BelWue.DE. 994 SOA dns1.BelWue.DE. hostmaster.BelWue.DE. 2026030609 21000 1800 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0