Figure 1 shows what protocol the Mailbox server uses to communicate with each of these roles or computers. Each numbered interaction in Figure 1 corresponds to the following list, describing what types of information is shared between these roles and computers.

Note:
If more than one server role coexists on a single computer, the server roles still use the protocols described in Figure 1 to communicate. However, the communication is internal to a single computer instead of traveling across your network to reach a different computer.

1. The Mailbox server accesses recipient, server, and organization configuration information from Active Directory.
   
   
2. The Store driver on the Hub Transport server places messages from the transport pipeline into the appropriate mailbox. The Store driver on the Hub Transport server also adds messages from the Outbox of a sender on the Mailbox server to the transport pipeline.
   
   
3. The Client Access server sends requests from clients to the Mailbox server, and returns data from the Mailbox server to the clients. The Client Access server also accesses offline address book files on the Mailbox server through NetBIOS file sharing. The types of data that the Client Access server sends between the client and the Mailbox server are messages, free/busy data, client profile settings, and offline address book data.
   
   
4. The Unified Messaging server retrieves e-mail and voice mail messages and calendar information from the Mailbox server for Outlook Voice Access. The Unified Messaging server also retrieves storage quota information from the Mailbox server.
   
   
5. Outlook clients that are inside your firewall can access a Mailbox server directly to send and retrieve messages. Outlook clients outside the firewall can access a Mailbox server using remote procedure call (RPC) over Hypertext Transfer Protocol (HTTP).
   
   

   Note:
   To send free/busy information and client profile settings between an Outlook client and a Mailbox server, you must have the Client Access server role installed. This information cannot be passed directly between the Outlook client and the Mailbox server.

6. The administrator-only computer retrieves Active Directory topology information from the Microsoft Exchange Active Directory Topology service. It also retrieves e-mail address policy information and address list information.
   
   

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