mx1.mk.de has no CNAME records.
Question and response
Question
dig mx1.mk.de. NS
Answer
The response has no answer records.
Authority
mk.de. 294 SOA ns1.mk.de. hostmaster.mk.de. 2026022005 10800 3600 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0