First I’d like to say that I absolutely love eM Client. Coming from Outlook was easy but it’s just so much better. I have imported all of my saved emails and customized the UI almost to where I want it.
The only thing left is, I would l like to turn off the preview pane in the Notes view just like I did in the Mail view. Is this possible, if so how?
Well I discovered the solution to this issue.
Settings -> Appearance -> Layout: Messages panel off
This causes the messages panel in Mail to turn off but not the preview in Notes. Change the theme and restart. The preview in the Notes will also be gone.
Technically I guess this would be a bug but the work around works for me.
I’d say that would need to be a “feature request” in settings, as is currently by design as far as I know that you cannot turn off notes preview separately. Most peeps do want to preview notes when they open them.
However if there is a way separately to disable the preview only in notes and not mail then someone I’m sure will update this thread.
It doesn’t work like that in version 9. 
The Notes preview is not affected by the message preview setting.