mx.nts-online.net has no NS records.
NS records for this zone are defined at nts-online.net.
NS records for this zone are defined at nts-online.net.
Question and response
Question
dig mx.nts-online.net. NS
Answer
The response has no answer records.
Authority
nts-online.net. 300 SOA dns1.nts-online.net. admin.nts-online.net. 638182471 7200 1200 3600000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0