mx.mail.rice.edu has no NS records.
NS records for this zone are defined at mail.rice.edu.
NS records for this zone are defined at mail.rice.edu.
Question and response
Question
dig mx.mail.rice.edu. NS
Answer
The response has no answer records.
Authority
mail.rice.edu. 300 SOA ns1.rice.edu. hostmaster.rice.edu. 2016013130 14400 3600 1200000 3600
Additional
. 0 OPT ; payload 1220, xrcode 0, version 0, flags 0