mx.paubox.com has no NS records.
NS records for this zone are defined at paubox.com.
NS records for this zone are defined at paubox.com.
Question and response
Question
dig mx.paubox.com. NS
Answer
The response has no answer records.
Authority
paubox.com. 900 SOA ns-790.awsdns-34.net. awsdns-hostmaster.amazon.com. 2 7200 900 1209600 90
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0