Outlook stores email in Microsoft Cloud - what you need to know - Runbox Blog blog.runbox.com

The new Outlook acts more like a middleman, and the app is now more like a web-based client than a traditional desktop program. It no longer logs directly in to the Runbox servers, but rather uses a web connection (HTTPS) via Exchange Web Services (EWS) or Outlook Web Access (OWA). This means that Microsoft’s own cloud service logs in to your email account, stores a copy of your emails, and then delivers that data to your Outlook app.