maildomain.datevnet.de has no CNAME records.
Question and response
Question
dig maildomain.datevnet.de. NS
Answer
The response has no answer records.
Authority
datevnet.de. 600 SOA ns01.datev.de. dnsmaster.datev.de. 2602260038 21600 3600 604800 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0