As Apple do not provide the facility to delete ALL the emails in your idevice in box, here is a quick and simple workaround to 'select all' and 'delete all' the in box email.
***Update From Feedback***
If you have many emails to remove then please try holding the move button longer, say 10sec per 250 emails.
I think it may work more reliably on the iPad compared to iPhone or iPod.
This will work fine on a standard non jail broken device.
Step by step for many deleting e-mails 200+.
1. Watch the video
2. When you Edit - Select - Hold Move - Deselect
Keep holding Move for 10 to 30 sec if you have between 200 and 1000 e-mails to move
3. WAIT
On then next screen when shown all the folders wait until all the emails are stacked, typically up to 30 sec for a lot of emails.
4. Select Trash folder
5. Go to trash and delete all as in the video.
