mx1.debeka.de has no CNAME records.
Question and response
Question
dig mx1.debeka.de. NS
Answer
The response has no answer records.
Authority
debeka.de. 663 SOA dns-dmz-ext2.extranet.debeka.de. hostmaster.debeka.de. 2022052824 10800 3600 2419200 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0