There are several reasons. A few are:

1. Opportunity for Server Consolidation From Exchange 5.5 and Exchange 2000 because you can get more mailboxes on an Exchange 2003 Server.

2. Better security features. The server is secure by default and has added things like automatic logoff for an inactive OWA session, Connection filtering, and has more junk mail features like real-time blacklists.

3. Availability enhancements such as End-to-End Outlook Monitoring, Improvements in ESM, Mailbox Recovery Center, and a Recovery Storage Group.

4. Increase in Mobile device support for Pocket PC’s, Pocket PC Phones and Microsoft Windows®-powered Smartphones.