mx1.checkdomain.de has no CNAME records.
Question and response
Question
dig mx1.checkdomain.de. NS
Answer
The response has no answer records.
Authority
checkdomain.de. 151 SOA ns.checkdomain.de. hostmaster.checkdomain.de. 2026030401 40000 7200 604800 180
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0