outbound6.paubox.com has no MX records.
Question and response
Question
dig outbound6.paubox.com. MX
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