Both the iPhone and iPod Touch can link to e-mail accounts and "push" notifications of incoming mail. This means that instead of having to load your e-mail inbox to see whether you have received anything the iPhone will play a sound and display the number of e-mails received as soon as they arrive. There are a few common set-up errors that could cause push notification problems.
The push notification feature on an iPhone only works with MobileME, Microsoft Exchange and Yahoo! e-mail accounts. Other accounts, even those as widely used as Gmail, AOL and POP (which includes Hotmail) are not supported. If your e-mail account is incompatible, you will either have to set up a new one or do without real-time notifications.
To extend battery life, the iPhone is able to set a delay between checking your e-mail account. This could mean that your phone could be set to allow a delay of up to an hour between your e-mail account receiving mail and checking for a notification. To adjust the fetch delay tap the "Settings" icon on the home screen, then tap "Fetch New Data." Tap the "Advanced" button at the bottom of the screen, then your e-mail account. Set the account to "Push." This maintains a constant connection to the account which uses more battery but gives instant notification of incoming e-mails.
Update Your Phone
Your iPhone needs to be running at least iOS 4.2 or later to enable push notification. Updating your phone can also solve myriad unexplained problems, and it's free so there is no reason to not update. Download and install the latest version of iTunes available at apple.com. Connect your phone to the computer then start iTunes and click "Check for updates" under the summary tab at the top of the screen. Follow the on-screen instructions to install any available updates to your phone.
Resync Your Account
If you are sure that your e-mail account is compatible with the iPhone and that push notification is not working, Apple recommends that you resync the email account to the phone. From the home screen tap "Settings," then "Mail." Retype the information the iPhone requests to reset the account association.