mailbox02.ventari.de has no CNAME records.
Question and response
Question
dig mailbox02.ventari.de. NS
Answer
The response has no answer records.
Authority
ventari.de. 717 SOA ns-230.awsdns-28.com. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0