To delete all emails on an iOS device, select the Edit button in the inbox, and tap a single email to highlight it. While holding down the Move button, deselect the email, and then release both fingers and tap the trash folder to move all emails into the trash.Continue Reading
Tap the Mail application on the iPhone home screen to access your email. Select the Inbox option to view and edit the contents of your email inbox.
Tap the Edit button, and select any one email to highlight it. Place and hold a finger on the Move button. While holding that finger down, use another to deselect the highlighted e-mail. Release both fingers to select all emails in the inbox.
While all emails remain selected, tap the Trash folder icon to move all of the contents of the Inbox folder to the Trash folder.
Tap the Trash folder icon once again to view the contents of the folder. Select the Delete All button to empty the Trash folder of all emails and permanently remove them from your iPhone.