mail03.u2d.de has no NS records.
NS records for this zone are defined at u2d.de.
NS records for this zone are defined at u2d.de.
Question and response
Question
dig mail03.u2d.de. NS
Answer
The response has no answer records.
Authority
u2d.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