dispatchb-usg1.ppe-hosted.com has no MX records.
Question and response
Question
dig dispatchb-usg1.ppe-hosted.com. MX
Answer
The response has no answer records.
Authority
ppe-hosted.com. 300 SOA ns1.proofpoint.com. ops.proofpoint.com. 2017042014 1800 900 2419200 300
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0