Microsoft announced on 9/20 that iPhone and iPad users of the built-in Apple Mail app may be unable to sync their Office 365 mailbox or login to their accounts after upgrading to the new iOS 11.
Microsoft suggests to avoid updating to iOS 11 until the issue has been fixed. If you've already upgraded, Microsoft recommends installing the free Outlook for iOS client from the App Store.
Microsoft further suggests that users check their support article for updates to the problem. Microsoft and Apple are working together to resolve the problem.
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