Assume that accessing external Exchange Web Services (EWS) URL is blocked from internal Exchange servers in a Microsoft Exchange Server 2016 or Exchange Server 2013 environment. In this situation, the client applications that are posting ExecuteEwsProxy calls will be failed to connect, such as Salesforce add-in for Outlook and Outlook Web App (OWA).
This issue occurs because the ExecuteEwsProxy function is only using external EWS URL. This causes the makeEwsRequestAsync method fail to call EWS.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.