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If "Mark items as read when viewed in the Reading Pane" and e.g. "Wait 5 seconds before marking items as read" is selected, messages are marked as read if you stay 5 seconds or
longer in the same message.
If "Mark item as read when selection changes" is selected, messages are marked as read when they are browsed with the mouse.
If "Mark item as read when selection changes" is not selected, messages are not marked as read before they are opened by double-clicking.
If "Single key reading using space bar" is selected, you can read messages by pressing the space bar.
