mx01.dg-w.de has no CNAME records.
Question and response
Question
dig mx01.dg-w.de. NS
Answer
The response has no answer records.
Authority
dg-w.de. 10456 SOA dnsauth001.dg-w.de. hostmaster.deutsche-glasfaser.de. 2026030501 10800 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0