ns-at1.proofpoint.com has no MX records.
Question and response
Question
dig ns-at1.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