By default, the iPhone will only check for new emails manually when you open the Mail screen or when you tap the refresh icon (the icon in the lower left screen of the email list). You can configure your iPhone to automatically check for new emails by following these easy steps.
Tap on the Settings icon in the home screen and tap the Mail button.
In the new screen that slides over, look for the Auto-Check option in the Messages section.
Tap the Auth-Check option and select the frequency of updates from the list that will slide over. On release day, the available choices are Manual, Every 15 Minutes, Every 30 Minutes, and Every Hour.