How to Delete All of the Mail in My iPhone's Inbox
The iPhone's Mail app contains a number of features that allow you to organize and manage the messages in your email inbox. By default, swiping your finger across any message in your inbox reveals an "Archive" button, but you can configure this function to reveal a "Delete" button instead. This delete function works well if you have just a few messages to delete, but the best way to delete all of the messages in your inbox is to use the Mail app's batch delete process.
Press the "Home" button on your iPhone to access the home screen.
Tap the "Mail" app icon.
Tap "Inbox" in the main menu if the Mail app does not automatically open your inbox.
Tap the "Edit" button in the top-right corner.
Tap each of the messages in your inbox to add a check mark beside each message. Tap the "Delete" button at the bottom to delete all of the messages in your inbox.