If your phone software is reinstalled, you will have to reinstall Mail for Exchange, just like any other app that you have installed.
If your phone software is updated, you do not have to reinstall Mail for Exchange. When your phone receives an over the air update, the Mail for Exchange app and its data are restored by the update.
Note: The Mail for Exchange app is available via a preload application for Nokia 301, Nokia 301 Dual SIM, Nokia 515 and Nokia 515 Dual SIM phones. Simply start the Mail for Exchange app in the phone and the phone will automatically download the full package of the Mail for Exchange client.