One of my real irritations with mail in the Notes client, especially with the three-column view in Notes 8.x.
When you have focus on the message itself in the preview pane, rather than on the message in the inbox view, the delete shortcut (e.g. backspace on the Mac) ceases to operate. Therefore if you scroll down to see the bottom on the message then wish to delete, you have to click back to the inbox before it will work.
Therefore, my idea - ensure the delete shortcut works wherever you are in the mail client if viewing a message.