mx01.ionos.de has no NS records.
NS records for this zone are defined at ionos.de.
NS records for this zone are defined at ionos.de.
Question and response
Question
dig mx01.ionos.de. NS
Answer
The response has no answer records.
Authority
ionos.de. 600 SOA ns-1and1.ui-dns.com. dnsadmin.1and1.com. 2018063107 14400 3600 605000 600
Additional
. 0 OPT ; payload 1220, xrcode 0, version 0, flags 0