Here's a simple summary of Exchange 2007 server roles, taken from http://msexchangeteam.com/archive/2006/09/12/428880.aspx
- Mailbox (MB): responsible for hosting mailbox and public folder data.
- Client Access (CA): provides mailbox server protocol access (but not MAPI). Similar to Exchange 2003 FrontEnd server, it enables user to use OWA, POP3/IMAP4 and mobile device (ActiveSync) to access their mailbox.
- Hub Transport (HT): handles mail routing to the next hop: another Hub Transport server, Edge server or mailbox server. Unlike Exchange 2003 Bridgehead that needs Exchange admin defined routing groups, Exchange 2007 Hub Transport role uses AD site info to determine the mail flow.
- Edge Transport (ET): The last hop of outgoing mail and first hop of incoming mail, acting as a "smart host" and usually deployed in a perimeter network (DMZ), Edge Transport provides mail quarantine and SMTP service to enhance security. One advantage of this role is that is does not require Active Directory access, so it can function with limited access to the corporate network for increased security.
- Unified Message (UM): enables end users to access their mailbox, address book, and calendar using telephone and voice. IP-PBX or VoIP gateway needs to be installed and configured to facilitate much of the functionality of this server role.