ns4.proofpoint.com has no MX records.
Question and response
Question
dig ns4.proofpoint.com. MX
Answer
The response has no answer records.
Authority
proofpoint.com. 1200 SOA ns1.proofpoint.com. ops.proofpoint.com. 2018114806 28800 7200 604800 1200
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0