This might be helpful –

https://ipcheck.proofpoint.com/

From: NANOG <nanog-bounces@nanog.org> On Behalf Of Tim Donahue
Sent: Thursday, 10 January 2019 11:21 AM
To: nanog@nanog.org
Subject: Proofpoint Mail Delivery Issues

Hi all,

Sorry for the noise, but one of my clients is getting the standard “it’s the other guy’s fault” with some email delivery issues to/from Proofpoint “Enterprise” customers.  If there is anyone from Proofpoint support monitoring this list, some assistance troubleshooting email delivery issues would be greatly appreciated.

Thank you,

Tim Donahue