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Outlook App for iOS and Android does not connect directly to your corporate Exchange server. Instead, it connects to a server or service hosted in the cloud by Microsoft. When you configure an account in Outlook App on iOS and Android, the app connects to the Microsoft cloud service and the cloud service connects to your mailbox on your on-premises Exchange server, on your behalf using your user credential. The cloud service will store your user credential and cache your email data. Effectively, the Microsoft cloud service acts as a middle man.
Not only that you might not trust Microsoft to securely store and safeguard your data including your credentials, but also, providing your user credentials to a third party is a breach of many IT usage policies. The Outlook app doesn’t even make clear to end users that this is occurring. In fact, the typical end user would have no idea that this is happening.
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