mx.paubox.com has no CNAME records.
Question and response
Question
dig mx.paubox.com. NS
Answer
The response has no answer records.
Authority
paubox.com. 356 SOA ns-790.awsdns-34.net. awsdns-hostmaster.amazon.com. 2 7200 900 1209600 90
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0