More than ever, the iPhone is a real-time window into the data stream of your life. Whatever changes are made to your calendar, address book, or email back on your computer at home (or at the office) can magically show up on your iPhone, seconds later, even though you're across the country.
That's the beauty of so-called push email, contacts, and calendars. You get push email if you have a free Yahoo Mail account (Chapter 8).You get all three if you've signed up for a MobileMe account (Chapter 14), or if your company uses Microsoft Exchange (Chapter 15).
Having an iPhone that's updated with these critical life details in real time is amazingly useful, but there are several reasons why you might want to use the Off button here. Turning off the push feature saves battery power; saves you money when you're traveling abroad (where every "roaming" Internet use can run up your AT&T bill); and spares you the constant "new mail" jingle when you're trying to concentrate (or sleep).
Tip
If you tap Advanced, you can specify either "push" (real-time syncing over the air) or "fetch" (checking on a schedule) for each type of program: Mail, Contacts, and Calendars, for each account you have.
And what if you don't have a push email service, or if you turn off "push?" In that case, your iPhone can still do a pretty decent job of keeping you up to date. It can check your email once every 15 minutes, every half hour, every hour, or only on command (Manually). That's the decision you make in the Fetch panel here. (Keep in mind that more frequent checking means shorter battery life.)
Tip
The iPhone always checks email each time you open the Mail program, regardless of your setting here. If you have a "push" service like MobileMe or Exchange, it also checks for changes to your schedule or address book each time you open Calendar or Contacts—again, no matter what your setting here.