mx2.proofpoint.com has no CNAME records.
Question and response
Question
dig mx2.proofpoint.com. NS
Answer
The response has no answer records.
Authority
proofpoint.com. 930 SOA ns1.proofpoint.com. ops.proofpoint.com. 2018114806 28800 7200 604800 1200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0