mx1.belwue.de has no CNAME records.
Question and response
Question
dig mx1.belwue.de. NS
Answer
The response has no answer records.
Authority
belwue.de. 3600 SOA dns1.belwue.de. hostmaster.belwue.de. 2026022706 21000 1800 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0