Due to an incompatibility in the new release of iOS (released 9/19), users of the built-in Mail app in iOS 11 may be unable to sync their WPI (Office 365) mailbox or log in to their accounts. iOS 9 or 10 users are not affected. Microsoft and Apple are working to resolve this issue. More information will be posted as it becomes available.
Access to WPI (Office 365) mailbox is unavailable for iOS 11 users.