How can I "Delete All" from Inbox or Sent mail folder in iOS?
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 E-Mail
  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 Trahs-Folder, you can delete all items at once