mail06.secure-mx.de has no CNAME records.
Question and response
Question
dig mail06.secure-mx.de. NS
Answer
The response has no answer records.
Authority
secure-mx.de. 515 SOA dns.dns1.de. hostmaster.partnergate.com. 2025030301 28800 7200 604800 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0