mailbox01.ventari.de has no NS records.
NS records for this zone are defined at ventari.de.
NS records for this zone are defined at ventari.de.
Question and response
Question
dig mailbox01.ventari.de. NS
Answer
The response has no answer records.
Authority
ventari.de. 300 SOA ns-cloud-a1.googledomains.com. cloud-dns-hostmaster.google.com. 1 21600 3600 259200 300
Additional
. 0 OPT ; payload 512, xrcode 0, version 0, flags 0