mx.firushima.com has no NS records.
NS records for this zone are defined at firushima.com.
NS records for this zone are defined at firushima.com.
Question and response
Question
dig mx.firushima.com. NS
Answer
The response has no answer records.
Authority
firushima.com. 3600 SOA dns.firushima.com. hostmaster.firushima.com. 1538937778 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0