mx00.kundenserver.de has no CNAME records.
Question and response
Question
dig mx00.kundenserver.de. NS
Answer
The response has no answer records.
Authority
kundenserver.de. 577 SOA rns.ui-dns.de. dnsadmin.1und1.de. 2013099672 10800 3600 604800 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0