mail.netcup.de has no NS records.
NS records for this zone are defined at netcup.de.
NS records for this zone are defined at netcup.de.
Question and response
Question
dig mail.netcup.de. NS
Answer
The response has no answer records.
Authority
netcup.de. 7200 SOA root-dns.netcup.net. dnsadmin.netcup.net. 24 28800 7200 1209600 7200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0