mx.xservers.net has no NS records.
NS records for this zone are defined at xservers.net.
NS records for this zone are defined at xservers.net.
Question and response
Question
dig mx.xservers.net. NS
Answer
The response has no answer records.
Authority
xservers.net. 1200 SOA nsa.xservers.ro. tehnic.xservers.ro. 2025112608 3600 1800 1209600 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0