The 'Message Pane' can be displayed by:

On the 'Menu Bar' toolbar 'View' > 'Layout' > select 'Message Pane'

Or quick method - toggle the view using f8 key

or if using the menu icon Menu Icon > Options > select 'Menu Bar' to enable it.

Just as a footnote: There is no 'Preview' pane. Those words imply that you get to see a 'preview' of an email before opening to read.

The 'Message Pane' shows a selected opened email.

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