7/11/2019, 2:30 PM, UPDATE: The certificate issue has been resolved.
7/11/2019, 9:00 AM: Since the recent upgrade of our Exchange hybrid server, many Outlook client users are experiencing certificate warnings that pop-up as security alerts. The alerts may continue to pop up whether you click "Yes" or "No." Staff are working to resolve the certificate issue so the pop-ups will cease. These pop-ups affect Outlook for Windows and macOS, but not the mobile Outlook app or the Outlook web app.