How can I "Delete All" from Inbox or Sent mail folder in iOS?

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

This worked for me:

1. Select the Sent Items folder
2. Select Edit
3. Select Select All (all the sent e-mails will be highlighted)
4. Press the Delete key (all the highlighted e-mails will move to the Deleted Items folder
5. • a) Right click on the Deleted Items folder
   • b) Select Empty 'Deleted Items' folder

Or

6. If your Outlook is set-up to automatically delete the items in the Deleted Items folder, they will auto delete when you close Outlook.