Exchange 2007 has asset of five transport roles. All but one of these can be deployed on the same server. The Mailbox role is straightforward: It allows a server to host mailbox and public folder data and provides MAPI access for Outlook clients. The Client Access role provides client access for everything other than MAPI (POP3, OWA, and IMAP). The Unified Messaging role is used with IP-PBX or VoIP gateway software and combines mailbox, address box, and calendaring with voice. The Hub Transport role allows a server to function as a mail router. The Edge Transport server is deployed by itself and is usually put on a server sitting on an organization's DMZ.