mx.900056.com has no NS records.
NS records for this zone are defined at 900056.com.
NS records for this zone are defined at 900056.com.
Question and response
Question
dig mx.900056.com. NS
Answer
The response has no answer records.
Authority
900056.com. 3600 SOA dns.900056.com. hostmaster.900056.com. 1657650615 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0