Many Organizations aren't ready to upgrade their entire Enterprise to Exchange 2007 however they do want to take advantage of the new Mobile features and policies offered by Exchange 2007. Many of these new management and monitoring features can be obtained by upgrading only the Exchange 2003 SP2 Fontend Servers to Exchange 2007 FrontEnd or CAS servers unfortunately you won't get any of the new security or usability enhancements. While this is not the recommended deployment practice by Microsoft (recommended Deployment steps are outlined below stating that Microsoft recommends that all Servers in a Site run the same server version) it is supported.
Important: Using an Exchange Server 2003 front-end server together with an Exchange 2007 Mailbox server is not supported.
The Client Access server role provides the functionality that was provided by a front-end server in Exchange 2003. The Client Access server role can coexist with Exchange Server 2003 and Exchange 2000 Server servers. The following list describes the Exchange 2007 dependencies and requirements for coexistence with Exchange Server 2003 and Exchange 2000 Server.
Exchange ActiveSyncThe version of Exchange ActiveSync that clients use also depends on the server version that is hosting the user's mailbox. When a client connects to the Exchange 2007 CAS server we check to see where the user is located. If they are on a 2003 Mailbox Server we use the E2K3 version of the Activesync Protocol, if their mailbox is on a Exchange 2007 Mailbox Server then we pass on the connection to the Mailbox server where they use the new version of Activesync with the device. So a user whose mailbox is located on an earlier server version will be unable to use new features, such as Sharepoint/UNC access and Exchange Search as the older version of the Activesync Protocol doesn't support these requests.
Note: Exchange Search and other features and policies must be supported by the device to work. Currently Windows Mobile 5 doesn't support addition policies and features that were not present in Exchange 2003 SP2.
When you perform a transition from Exchange Server 2003 to Exchange 2007, you will typically transition all the Exchange servers in a particular routing group or Active Directory site to Exchange 2007 at the same time, configure coexistence, and then transition the next site. If you are installing the server roles on separate hardware, Microsoft recommends that you deploy the server roles in the following order:
What will I get by just upgrading the Front End Server to a Exchange 2007 CAS server?
What features require a Exchange 2007 CAS server as well as a Exchange 2007 Mailbox Server
Outlook Web AccessWhether a user sees the Outlook Web Access client of Exchange Server 2003 or Exchange 2007 depends on the location of the user's mailbox. For example, if the user's mailbox is located on an Exchange Server 2003 back-end server and the Client Access server is running Exchange 2007, the user will see the Exchange Server 2003 version of Outlook Web Access.
The Outlook Web Access URL used to access the logon dialog depends whether the user's mailbox is located on an Exchange 2003 back-end server or on an Exchange 2007 Mailbox server. Use one of the following URLs to reach Outlook Web Access: If the mailbox is located on an Exchange Server 2003 back-end server, use http://<servername>/Exchange. If the mailbox is located on an Exchange 2007 Mailbox server, use either http://<servername>/owa or http://<servername>/Exchange.
The Outlook Web Access URL used to access the logon dialog depends whether the user's mailbox is located on an Exchange 2003 back-end server or on an Exchange 2007 Mailbox server. Use one of the following URLs to reach Outlook Web Access:
Important: Before you configure Client Access servers and decommission your Exchange Server 2003 front-end servers, determine whether you want to retain any Outlook Web Access settings or custom configurations, security updates, themes, and customization configurations, from your Exchange Server 2003 front-end servers. Installation of Exchange 2007 requires 64bit hardware, and no settings or custom configurations from Exchange Server 2003 are retained. Therefore, before you decommission your front-end servers and install Client Access servers, make sure that the Outlook Web Access settings and custom configurations on your Exchange Server 2003 back-end servers match the configurations on your Exchange Server 2003 front-end servers.
Outlook Mobile AccessExchange Activesync is often confused for OMA which is Outlook Mobile Access. Outlook Mobile Access is similar to Outlook Web Access, but with a more simplistic, non-graphical interface designed for Mobile Browsers on all devices (Nokia, Palm OS etc). Activesync is the service that syncs PIM (Email, Calendar, Contacts and Tasks) information with the Mobile Outlook Client on Windows Mobile Devices. Outlook Mobile Access has been removed in Exchange 2007.
Important: Outlook Mobile Access (Mobile Browser based Email Access) has been removed in Exchange 2007
Information obtained from:http://technet.microsoft.com/en-us/library/aa998186.aspx