How to Delete All Mail on iPhone
You can delete emails from the iPhone one by one, delete multiple messages at once or remove entire accounts worth of messages.
You can delete individual email messages from the iPhone's Mail app, delete multiple email messages at once or remove an entire email account from the phone.
Deleting Individual Emails
Tap the Mail icon on your home screen to launch the Mail app.
To delete a message without touching it, swipe left across it so the Trash button appears.
Touch the Trash button to delete the message.
If you prefer to read the message before deleting it, tap it in the Inbox menu.
Within the individual message's screen, touch the Delete icon to delete the message.
To delete messages from another email account, or from a mailbox or folder besides your inbox -- such as a sent mail folder or junk mail folder--touch the Mailboxes button in the Mail app.
Many email programs use the term "folder" in the same way that Apple uses "mailboxes." If you're used to accessing your inbox, sent mail and other collections of email in a list of folders in another program, you'll find them listed as mailboxes in the Mail app.
Touch an account name if you access multiple email accounts on your iPhone.
Within the Mailboxes menu, touch an individual mailbox or folder to view and delete messages within it.
Deleting Multiple Messages
To delete multiple messages from an email mailbox or folder in the Mail app, touch its name in the Mail app's Mailboxes menu.
Touch the Edit button within the mailbox screen.
Touch the select box next to each email message you want to delete.
Touch the Trash button to delete the messages you've selected.
Deleting Entire Mail Accounts
You can delete an email account from your iPhone entirely.
Touch the Settings icon on your phone's home screen.
Within the Settings menu, touch the Mail, Contacts, Calendars icon to access the mail settings.
Touch the name of the account to delete.
To remove mail from the account from your phone while still retaining access to other data from the account, such as calendar reminders, touch the Mail toggle button.
To delete the account entirely, touch the Delete Account button.