Currently, it is a minor hassle to select multiple messages. When you receive many messages at once, it’s useful to be able to mark them all as read:
It would be a small user experience improvement to be able to select each checkbox the same way most websites allow by selecting multiple at the same time while holding Ctrl, like so (Gmail):
It would also improve UX if the collision box for each checkbox was expanded. In this image, the red box represents where the user clicks in order to select a message:
