Via Menu Bar View > Layout > Vertical view

Via Mail Toolbar Menu icon > Options > Layout > Vertical View

Just as a note - 'preview pane' implies you are seeing a 'preview' of something that is not opened. I have come across some people in the past who thought it really was an unopened preview, so I thought I would just mention it for clarity.

You are referring to the 'Message Pane'. When you select an email in the list and you can see the contents in the 'Message Pane' below the list of emails. That email is not a preview, it is an opened email.

You can open an email by: single click to read in Message Pane double click to open to read in tab or window.

Both methods are an opened email, just different options. Larger emails can be easier to read in a tab, but smaller ones are easy and quick to use the Message Pane.

Thanks for the help. The Layout options are disabled.

What does that mean?

Problem is solved.