mx1.isp.ktk.de has no CNAME records.
Question and response
Question
dig mx1.isp.ktk.de. NS
Answer
The response has no answer records.
Authority
ktk.de. 1603 SOA ns-a.ktk.de. hostmaster.noc.ktk.de. 2019055355 3600 900 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0