The Reading Pane shows a preview of the selected email without opening it in a separate window.
If the preview has disappeared or moved, you can change its position from the View menu.
Open Outlook and go to:
View > Reading Pane
Choose one of the following:
Right - Displays the selected email on the right-hand side.
Bottom - Displays the selected email underneath the message list.
Off - Turns the Reading Pane off completely.
For the usual Outlook layout, select:
Right
Changing the Reading Pane does not move or delete any emails. It only changes how messages are displayed.