How to Reset Email Settings on the iPhoneby Stephen Lilley
When you first bought your iPhone, no email account was set up for the built-in Mail application. Therefore, if you want to reset your iPhone's email settings back to the way they were when you initially got the device, you have to go into the "Settings" application and delete any account you've added to your phone. This doesn't affect the rest of the data on your iPhone.
Turn on your Apple iPhone using either the "Sleep/Wake" button on the top of the device or the "Home" button on the front of the device.
Tap the "Settings" application icon.
Tap the "Mail, Contacts, Calendars" menu item.
Tap the name of the account listed in the "Accounts" section of the menu on screen. Scroll to the bottom of the "Account" page and select "Delete Account." At the confirmation prompt, select "Delete Account" again.
Return to the "Accounts" page using the "Back" button at the top of the screen and delete any additional accounts that are listed on screen. This will reset your iPhone's mail capabilities back to the way they were set up when you originally purchased your device.