I noticed that you can delete multiple message simultaneously by selecting them, upon which a hovering menu presents itself on the right side of the screen with some options. However, this menu does not appear when a message is being previewed in the preview pane. I wonder if this is by design or a bug.
The best use case would be to show the hovering menu every time a message is being selected in the selection tick box, irrespective of whether another email is being previewed. I don’t see the risk of people misunderstanding which email they are deleting, since they would have to consciously select an email via the tick box. Another to make sure there is no confusing would be to have the hovering menu placed somewhere else, for example on the right side of the email list (and thus the left side of the preview pane).
Hope I have explained this well enough, I don’t really know a better way of describing that menu.