So, if you're in the same boat, using an iPhone, and want to dump all those messages I'll show you how to delete them all in one process without having to click on each one to select them.
(Pssst - this tip will let you bulk delete OR bulk move messages around in iOS 10)
- If there are any messages in the folder that you want to keep, move them to another folder.
- Once you're sure that only unwanted emails are left, tap "Edit" in the upper-right corner. Once in edit mode, select one email by tapping the circle to the left of it.
- Tap and hold the "Move" button located at the bottom of your screen, but don't let go just yet. tap the circle to the left of the email that you chose in the previous step to deselect it, now you can take your finger off the "Move" button. This selects all of the emails in the folder at once.
- Tap "Trash" on the next screen to move all of the emails to the Trash folder.
- To completely delete the messages, open the Trash folder, tap "Edit", and tap "Delete All".
That's all there is to it and the clutter is gone, Gone, GONE.