When clicking a message, it automatically shows in the preview pane. When deleting that message, the system shows the next message automatically, thereby marking it as read. The user then has to manually select the message from the list and mark it as unread. It's very time consuming. If we could disable the preview pane, that would solve the problem.

I realize that most mail software has this in place and it works this way, by design. However, it's not always useful. It would be great if we had the option to turn this preview pane on or off.

Horizontal layout is currently available:
https://afterlogic.com/docs/webmail-pro-8/configuring-webmail/how-to-enable-horizontal-layout

Preview pane, however, is always visible.