1 Open the Mail app in iOS 10
2 Go to the inbox or mailbox folder you want to delete all emails from
3 Tap on the “Edit” button in the upper right corner
4 Now manually tap on every single email you want to delete, as signified by having a blue check mark next to the email message
5 Now choose “Trash” in the corner
6 Confirm that you want to delete all selected emails by choosing “Trash Selected Messages”
Repeat with other folders and emails as necessary - a bit kludgey from prior method but it works.