Delete an Account
If you want to temporarily disable an account—so it doesn't appear on your accounts screen and Mail doesn't check it for new messages—you can easily turn it off (see On/Off). However, if you want to remove an account completely—meaning all settings and messages are permanently erased from your iPhone OS device (but not from the server or from other clients, in the case of IMAP or Exchange accounts)—you can delete it.
To delete an email account from your iPhone OS device, follow these steps:
Go to Settings > Mail, Contacts, Calendars.
Tap the name of the account you want to delete.
Tap Delete Account; in the confirmation alert that appears, tap Delete Account again.
The account is immediately removed from your device.