Issue
Yesterday, Microsoft Office365 suffered a major outage and mail stopped flowing there. Proofpoint dutifully spooled the email until O365 came back online. Situation has been resolved by Microsoft and everything now works as it should. We strongly recommend you subscribe to our status page (status.vircom.com) which an email address that isn't on O365 (GMAIL for instance) so you can get alerts should a future outage happen. All queued mail should be delivered by now.
For the bounced messages - unfortunately these emails were bounced by Office365 and in turn Proofpoint bounced them as well. You could resend them, all you need to do is search for mail sent yesterday and filter by the term "Bounced". Bulk select and then resend. This will attempt a resend of those bounced emails (do not navigate away from the page right away, wait a good 30 seconds)
Since the search only returns 1000 results at a time, you may need to repeat this operation until all the bounced messages were resent depending on volume
If your users were sending messages out TO office365 destinations on the outside world, they may have received temporary errors. In those cases those emails should've been delivered by now.