Efficient Tech

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.
Microsoft and Apple are working to resolve this issue and will post more information to the article, below, when it becomes available.
In the meantime, we recommend you download the free Outlook for iOS client, available in the App Store. Alternatively, you can avoid updating to iOS 11 until the issue has been fixed.
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Quote from a Microsoft article - in summary, if you are an Office 365 user and use an iPhone or iPad to read email using the built-in Mail app, either switch to Outlook (!) or hold off upgrading to iOS 11 until they get their stuff sorted.