You need to identify the order in which you will deploy Exchange Server 2007 servers

You are the messaging engineer for your company. Your network consists of a Microsoft Exchange Server 2003 organization that contains 20 servers. Users connect to their Exchange mailboxes through front-end servers by using Outlook Web Access and Outlook Anywhere.
You plan to transition from Exchange Server 2003 to Exchange Server 2007. You need to identify the order in which you will deploy Exchange Server 2007 servers. During the transition, your solution should allow users to access the messages that are stored in their mailboxes. What should you do?

A.
Install the Client Access server role, then the Hub Transport server role, and then the Mailbox server role. Move all mailboxes to the Mailbox server.

B.
Install the Mailbox server role and the Hub Transport server role concurrently. Move all mailboxes to the Mailbox servers, and then install the Client Access server role.

C.
Install the Mailbox server role and the Edge Transport server roles concurrently. Move all mailboxes to the Mailbox server, and then install the Client Access server role.

D.
Install the Edge Transport server role, and then install the Mailbox server role and the Hub Transport server roles concurrently. Move all mailboxes to the Mailbox server and then install the Client Access server role.

Explanation:
If you want to deploy the Exchange 2007 server roles on more than one computer, and if youre installing in an existing 2003 organization, we recommend you implement the roles in this order: CAS, Hub, Mailbox, UM server.

You can activate the Edge role at any time during the deployment phase. However, this server wont be fully functional until the Hub role is up and the Edge server is subscribed to the organization.

An Exchange 2003 FE server cant provide access to a 2007 Mailbox server. This machine cant transmit messages unless a Hub server also exists in its AD site. Therefore, when upgrading an existing organization, we recommend you install the CAS and Hub roles before the Mailbox server role.

http://technet.microsoft.com/en-us/library/aa997610.aspx