mx.rackspeed.de has no CNAME records.
Question and response
Question
dig mx.rackspeed.de. NS
Answer
The response has no answer records.
Authority
rackspeed.de. 575 SOA ns.rackspeed-dns.de. hostmaster.rackspeed.de. 2026022305 3600 600 1209600 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0