How Can I Delete ALL emails from my iPhone Sent Folder?
Try this:


  Go to your Sent folder
  
  Tap on the Edit Button on the top right
  
  Select the first email in your list
  
  Hold down the "Move" button
  
  While you are still holding the Move button, deselect the first email
  
  Put away all your fingers from the screen and wait a few seconds
  
  Now Mail asks you where to move All of your emails
  
  Select the Trash-Folder
  
  Inside the Trash Folder, you can delete all items at once


More details:


https://www.imobie.com/support/delete-all-emails-iphone-ipad.htm