mail.mail24server.de has no CNAME records.
Question and response
Question
dig mail.mail24server.de. NS
Answer
The response has no answer records.
Authority
mail24server.de. 584 SOA www.mail24server.de. root.mail24server.de. 2007052410 28800 7200 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0