If you attempt to use the default mail app in iOS 11, good luck. According to a new KB article (KB4043473), Outlook.com, Office 365, and Exchange Server 2016 running on Windows Server 2016 are not compatible.
Instead, Microsoft suggests downloading and using the Outlook for iOS app from iTunes.
Outlook client for iOS: https://itunes.apple.com/app/id951937596
The KB article suggests that Microsoft and Apple are working on a resolution, but the Outlook app for iOS is a much better, more elegant solution anyway.
Apple has also posted its own statement about this problem…
UPDATE: Some are reporting that they are having no issue with iOS 11 and the default Apple mail app. They are running hosted Office 365 Exchange accounts. So, who to believe? Microsoft and Apple, or the customers who are having no issue at all? If it is working for you, the backend you are connecting to does not fall within the limits imposed by the incompatibility or someone has performed the workaround shown in the KB article.
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