maildomain.datevnet.com has no CNAME records.
Question and response
Question
dig maildomain.datevnet.com. NS
Answer
The response has no answer records.
Authority
datevnet.com. 600 SOA ns01.datev.de. dnsmaster.datev.de. 2602240039 21600 3600 604800 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0