www.paubox.com has no MX records.
Question and response
Question
dig group16.sites.hscoscdn10.net. MX
Answer
The response has no answer records.
Authority
hscoscdn10.net. 1800 SOA jerry.ns.cloudflare.com. dns.cloudflare.com. 2398072987 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0