mx1.denic.DE has no CNAME records.
Question and response
Question
dig mx1.denic.DE. NS
Answer
The response has no answer records.
Authority
denic.de. 899 SOA ns1.denic.de. dns-operations.denic.de. 1772442319 10800 1800 3600000 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0