I'm not convinced this is a good feature idea as I don't see any value it adds to the game.
Hovering over attacks / reports was useful as a way of scanning it. However, mails should at least need to be manually opened before they can be 'hovered' over, to avoid interference with the 'read / not read' part of the mailing settings. When leading tribes, seeing who is / isn't opening mails and determining activity etc is important.