It would be really helpful if there was a counter that showed the total of the messages selected in the list window. The way I do this at the moment is to set the selection to 'Unread' which displays the figure in the Unread/Total at the top, then reset back to 'Read'. It's a bit clumsy, though and necessarily resets the read status for any messages within the selection which isn't ideal.

Showing the selection total in the right-click menu woud work, but better to display it after the Unread/Total at the top, suppressing it if no messages were selected.

This would be a really easy feature to add with no obvious downside, but a user option to enable/disable this could be added if deemed worthwhile.