The iPhone uses ActiveSync to sync your exchange mailbox’s Email, Contacts, and Calendar.

Important Note: When you configure a device to sync with Exchange, all existing calendar and contact information on the device is overwritten, it is recommend that you backup your data. Additionally, iTunes no longer syncs contacts and calendars with your desktop computer. You can still sync your device wirelessly with MobileMe services.

Procedure:

To synchronize your iPhone to your you@yourdomain.com exchange account, follow these instructions.

Go to Settings > Mail, Contacts, Calendars and then tap Add Account.

Tap Microsoft Exchange.

 Note, you can configure only one Exchange account per device

When you manually configure the device for Exchange, use this chart to identify the settings and information you enter:

Email:Enter in your Exchange accont
Domain:Enter in your Exchange Domain
Username:Enter your OMA address:
Password:Enter in your password
Description: 

Your iPhone (or iPod touch) will now try to locate your Exchange server using Microsoft’s Autodiscovery service. If the server cannot be located, the screen below is shown. Enter your Exchange server’s complete address in the Server field. Contact your Microsoft Exchange server administrator or hosted exchange provider if you are unsure of the address.

Email: Enter in your Exchange accont
Server:Enter in your Exchange accont
Domain:Enter in your Exchange Domain
Username:Enter your OMA address:
Password: Enter in your password
Description:

  Next, the device offers to immediately sync with the Exchange server. If you choose not to sync at this time, you can turn on calendar and contact syncing later in Settings > Mail, Contacts, and Calendars. By default, Exchange ActiveSync pushes new data to your device as it arrives on the server. If you prefer to fetch new data on a schedule or to only pull new data manually, use Settings > Fetch New Data to change the settings.

           
 If you wish to change how many days’ worth of emails are synchronized to your device, go to Settings > Mail, Contacts and Calendars. The default setting is three days.