mx.xtra.pl has no NS records.
NS records for this zone are defined at xtra.pl.
NS records for this zone are defined at xtra.pl.
Question and response
Question
dig mx.xtra.pl. NS
Answer
The response has no answer records.
Authority
xtra.pl. 86400 SOA ns1.xtra.pl. root.ns1.xtra.pl. 2025080101 10800 3600 1814400 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0