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Update: 09/27/2017: Apple has released a fix for this issue. Please update to iOS 11.0.1, or later. (https://support.apple.com/kb/HT204204)
On Tuesday, September 19, Apple released a new operating system, iOS 11. If you are using the native mail app on your iPhone or iPad, and upgrade to iOS 11, you may encounter issues.
How does this affect me?
Due to an incompatibility in the new release of iOS, users of the built-in Apple Mail app in iOS 11 may be unable to sync their Office 365 mailbox or login to their accounts. iOS 9 or 10 users are not affected.
What do I need to do to fix this?
Microsoft and Apple are working to resolve this issue. We will post more information here when it becomes available. In the meantime, it is recommended that you postpone the iOS upgrade until the issue is fixed. However, if you choose to upgrade, you can download the free Outlook for iOS client, available in the App Store, which will work with your Office 365 account.