mx.nt-do.de has no CNAME records.
Question and response
Question
dig mx.nt-do.de. NS
Answer
The response has no answer records.
Authority
nt-do.de. 58 SOA ns1.goneo.de. hostmaster.goneo.de. 2021030834 10800 3600 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0