When you're using the reading pane, you have control over when messages are marked as unread:
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In classic Outlook, on the File menu, select Options.
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Select the Advanced tab.
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Select Reading Pane.
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In the dialog, select Mark items as read when viewed in the Reading Pane.
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Specify how many seconds to wait before marking items as read.
To learn how to use the Reading Pane, see Change the Reading Pane settings.
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In new Outlook, select the View tab, then select
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Select Mail > Message handling.
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Under Mark as read, choose your preferred option.
