Outlook Application
When you turn notifications on you will receive an alert when you receive a new email message, a meeting request, or a task request.
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Select File > Options > Mail
Under Message arrival, select or clear the Display a Desktop Alert check box and then select OK.
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New Outlook or Outlook for the Web
If you're using Outlook in a web browser as your primary mail app, you may want to be notified of meeting reminders or incoming email messages.
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