If you are using Android 9 (Android Pie), a feature in the build may be restricting Outlook's background usage. In order to reenable Outlook alerts:
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Close and relaunch Outlook.
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Click VIEW on the notification regarding restricted background usage at the bottom of the screen.
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Click View Settings.
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Go to Manage Battery Usage.
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Find Outlook's settings, then go to Battery Usage or Use Details.
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If Background Restriction or Allow Background Activity is restricted, click on it to disable the restriction.
