Official update from proofpoint:
28 July 2020 22:15 UTC (13:15 PT)
SpamHaus has identified the issue as a False Positive on their system and corrected the matter. Reputation propagation will update for SpamHaus end-users which should resolve the mail flow interruption.
Looks like the proofpoint IP has been delisted by spamhaus (188.8.131.52) - have not received the official word from proofpoint yet but we've been monitoring closely here. Will post final update from proofpoint as soon as it is out. Thank you for your patience.
"We are currently aware of one of proofpoint's IPs as being blacklisted by spamhaus (sbl.spamhaus.org). The IP in questions is 184.108.40.206. Operations are working on remediation now.