Step 1: Upgrade to Exchange Server 2003 SP2
Exchange Server 2003 SP2 includes Exchange ActiveSync, the synchronization protocol that keeps the Exchange mailbox synchronized on client mobile devices. By default, Exchange ActiveSync is enabled.
Exchange Server 2003 SP2 contains new features that work with the Windows Mobile 5.0 Messaging and Security Feature Pack to help you to improve the deployment, security, and management of mobile devices.
To use mobile devices with the Windows Mobile 5.0 Messaging and Security Feature pack, you must upgrade your front-end Exchange server to Exchange Server 2003 SP2. Back-end Mailbox servers can remain at Exchange 2003 RTM or SP1. However, we recommend that you upgrade both front-end and back-end servers to take advantage of the updates in SP2.
How to Upgrade to Exchange Server 2003 SP2
Download the Service Pack 2 for Exchange Server 2003 file from the Microsoft Exchange Server TechCenter Web site.
Follow the directions provided to upgrade your Exchange servers to SP2.