proofpoint.us has no MX records.
Question and response
Question
dig proofpoint.us. MX
Answer
The response has no answer records.
Authority
proofpoint.us. 60 SOA ns1.proofpoint.com. ops.proofpoint.com. 2017042206 28800 2880 604800 1200
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0