What are the Microsoft Exchange Server 2007 server roles?

Mailbox. Hosts mailbox and public folder data. This role provides Messaging API (MAPI) access for Microsoft Outlook clients. Additionally a variation exists called the Clustered Mailbox role when hosting as part of a clustered environment; it requires no other Exchange roles running on the cluster instance

Client Access. Similar to the old front-end server role. Provides all other client protocol access, apart from MAPI (i.e., Outlook Web Access--OWA, POP3, IMAP, and ActiveSync)

Unified Messaging. Provides client access to mailbox, address box, and calendar via telephone and voice. Require Special IP-PBX or VoIP gateway software.

Hub Transport. Mail routing server that forwards mail to another hub transport server, edge server, or mailbox server. Unlike Exchange 2003, which uses Exchange routing groups, Exchange 2007 uses Active Directory (AD) sites.

Edge Transport. Gateway from the Exchange organization to the outside world. This is the last hop for outbound mail and the first hop for incoming mail. This is the server that provides mail quarantine. This role doesn't require AD access, making it ideal for perimeter deployment without opening up ports for AD access. This role must always be deployed on a server with no other Exchange roles.