Hi All,
Spoke to the Apple team on live chat, the "Delete all" function in emails was remove by mistake in iOS10 and should be fixed soon in a update. They gave me the following "work around" (not tried it yet) Hope this helps 🙂
So what you could do is go to your inbox, Tap Edit, then Mark All, and then Mark All as Read. This will then close the Edit mode.
Tap Edit again, and tap the top message in the inbox. Press and hold the Move button with your thumb.
While holding the Move button, uncheck the top message, and then release the Move button.
You will then be on a screen with a trash icon, click it and your emails are deleted.