there isnt a way to do this on the android itself. you can go to your inbox on your pc and mark all as read, once your phone syncs, they will appear as read on your droid. hope this helps, it was annoying me as well before i did this method.
In either gmail app or htc mail....I'd like to have the option to "select all" messages in a given folder - whether it is to mass delete/forward/reply, etc. I'm aware I can do it in the web based Gmail but I'm curio is that there appears to be no provision to do so in the htc mail app. Am I wrong?
Delete you can do by Menu-Delete-Menu-mark All- Delete
Delete you can do by Menu-Delete-Menu-mark All- Delete