mx1.rtl2.de has no NS records.
NS records for this zone are defined at rtl2.de.
NS records for this zone are defined at rtl2.de.
Question and response
Question
dig mx1.rtl2.de. NS
Answer
The response has no answer records.
Authority
rtl2.de. 600 SOA pfire-1.rtl2.de. postmaster.rtl2.de. 1771320521 600 600 86400 600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0