mx.coxmail.com has no NS records.
NS records for this zone are defined at coxmail.com.
NS records for this zone are defined at coxmail.com.
Question and response
Question
dig mx.coxmail.com. NS
Answer
The response has no answer records.
Authority
coxmail.com. 86400 SOA dukensumpov1z.services.cox.net. hostmaster.cox.net. 781212118 28800 7200 1382400 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0