When some emails are already selected, the current behavior of the check all box is to check all on the first click, and uncheck all on the second click. This is the opposite of the behavior in other email apps such as Gmail. If you already have some boxes checked, it's more likely that you've just finished whatever action you were taking and just need to deselect them all again, so the first click should uncheck all the boxes, and then a second click would check all the boxes. Or at least that's my opinion. If this ends up being controversial, it could maybe be a behavior that could be toggled in the settings.