re: How do I clear cache and temporary storage on iOS's native Email app?
Try going to the APP store. Once it loads tap any of the 5 icons on the bottom of the screen, Featured - Top Charts - Explore - Purchased - Updates, 9 or ten times. The screen will go blank and then reload. This is supposed to clear application cache/temporary file storage. Then at any screen hold the power button down until you see the Slide to Power off screen. Then hold the Home button for about 5 seconds until the Home screen comes back on. This is also supposed to clear temporary files.
It work for me on my iPhone 6S and iPad Air 2. I also do those steps after I use the Facebook APP to make sure everything is cleared out from that APP.
It seems to work very good.