mx.rexvona.com has no NS records.
NS records for this zone are defined at rexvona.com.
NS records for this zone are defined at rexvona.com.
Question and response
Question
dig mx.rexvona.com. NS
Answer
The response has no answer records.
Authority
rexvona.com. 3600 SOA dns.rexvona.com. hostmaster.rexvona.com. 1867117566 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0