mx.boreus.de has no CNAME records.
Question and response
Question
dig mx.boreus.de. NS
Answer
The response has no answer records.
Authority
boreus.de. 528 SOA ns1.boreus.de. hostmaster.boreus.de. 2026021352 14400 3600 604800 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0