mx.ext-dc.de has no CNAME records.
Question and response
Question
dig mx.ext-dc.de. NS
Answer
The response has no answer records.
Authority
ext-dc.de. 33 SOA ns.ext-dc.net. hostmaster.ext-dc.de. 2025120503 10800 3600 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0