mx.df.eu has no CNAME records.
Question and response
Question
dig mx.df.eu. NS
Answer
The response has no answer records.
Authority
df.eu. 2398 SOA ns.namespace4you.de. hostmaster.df.eu. 1760600697 16384 2048 1048576 2560
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0