Exchange 2007 Roles

Considerations
The first requirement to note is that Exchange Server 2007 must be deployed on 64-bit hardware and does not support an in-place upgrade from any previous version of Exchange. You must use the swing upgrade method to move your existing messaging services to Exchange 2007. Further, the Exchange organization must be operating in native mode to support the addition of Exchange 2007 servers, so if your organization is currently using Exchange 5.5, you will be required to perform an interim upgrade to Exchange Server 2003 before moving to Exchange 2007.
There are some important changes in Exchange 2007 you must consider when planning your transition.

For example, Exchange 2007 introduces role-based deployment, which lets you choose the messaging services you want to provide and deploy server roles specific to those services.

You can deploy the server roles individually on dedicated hardware, or install multiple roles on the same physical server, administered as separate entities

Exchange Server 2007 Server Roles
Mailbox Server

The Mailbox server role provides message storage for an organization. Exchange 2007 can support up to 50 stores per server. These stores can be deployed as 50 individual storage groups, or you can create up to 50 stores in a single storage group. The Mailbox server role is the only role that can be deployed as a cluster, so if you will be using clustering, you will need to install the Mailbox server on dedicated hardware.
Client Access Server

The Client Access server role replaces the functionality provided by a front-end server. It provides mailbox access to clients accessing Exchange using POP3, IMAP4, Outlook® Web Access (OWA), RPC over HTTPS (now known as Outlook Anywhere), and Exchange ActiveSync®.
Hub Transport Server

This role provides SMTP and MAPI message transport services for the Exchange organization. Every message that is sent or received by the users in your organization is processed by a Hub Transport server. This is great, because it ensures that no message can bypass the server-based rules or journaling policies that are provided by agents that fire at various points in the transport pipeline.
Unified Messaging Server

This role provides voice access to your mailbox. It integrates with your IP/VoIP gateway or IP-PBX to provide telephone access to messages and calendar items and lets you transcribe a reply. This role is new to Exchange, and is not able to interoperate with any previous versions of Exchange.
Edge Transport Server

The Edge Transport server is typically deployed in your perimeter network. It provides SMTP message transport between the Exchange organization and the Internet, and provides anti-spam and antivirus processing using transport agents. You can now standardize on a single technology for both your organizational and perimeter network servers. This seamless interaction model simplifies administration and allows for easy integration of perimeter servers.