My understanding is that Outlook for iOS uses a cloud service that talks to the Exchange server and stores your credentials&#x2F;data: <a href="https:&#x2F;&#x2F;blog.winkelmeyer.com&#x2F;2015&#x2F;01&#x2F;warning-microsofts-outlook-app-for-ios-breaks-your-company-security" rel="nofollow">https:&#x2F;&#x2F;blog.winkelmeyer.com&#x2F;2015&#x2F;01&#x2F;warning-microsofts-outl...</a>. So logging in involves giving Microsoft (and indirectly Amazon, since it runs on AWS), clear-text access to your information.