mailserver.martinahautau.de has no CNAME records.
Question and response
Question
dig mailserver.martinahautau.de. NS
Answer
The response has no answer records.
Authority
martinahautau.de. 1976 SOA ns5.kasserver.com. hostmaster.kasserver.com. 2603041155 28800 7200 1209600 7200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0