mx1.agenturserver.de has no NS records.
NS records for this zone are defined at agenturserver.de.
NS records for this zone are defined at agenturserver.de.
Question and response
Question
dig mx1.agenturserver.de. NS
Answer
The response has no answer records.
Authority
agenturserver.de. 3600 SOA ns01.agenturserver.de. support.mittwald.de. 1755875478 28800 7200 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0