mail02.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 mail02.u2d.de. NS
Answer
The response has no answer records.
Authority
u2d.de. 900 SOA ns-1956.awsdns-52.co.uk. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0