The Hub Transport server role processes all messages that are sent inside the Exchange 2007 organization before the messages are delivered to a recipient's Inbox or are routed to users outside the organization. There are no exceptions to this behavior; messages are always passed through a server that runs the Hub Transport server role.

Messages are submitted to the Hub Transport server in three ways: through Simple Mail Transfer Protocol (SMTP) submission, from the Pickup directory, or when a user inside the organization sends a message, and that message is picked up from the user's Outbox by the store driver, a software component of the Hub Transport server that delivers inbound messages to Exchange stores, the databases that contain public folder and mailbox stores.

When messages are submitted to the Hub Transport server, they are processed by the categorizer. The categorizer is a component of Exchange transport that processes all inbound messages and determines what to do with the messages based on information about the intended recipients. In Exchange 2007, the Hub Transport server uses the categorizer to expand distribution lists and to identify alternative recipients and forwarding addresses. After the categorizer retrieves full information about the recipients, it uses that information to apply policies, route the message, and perform content conversion. Messages are then delivered locally by the store driver to a recipient's mailbox, or they are delivered remotely by using SMTP to send messages to another transport server. Messages that are sent by users in your organization are picked up from the sender's Outbox by the store driver and are put in the Submission queue on a server that runs the Hub Transport server role. For more information, see Transport Architecture.

A collection of transport agents lets you configure rules and settings that are applied as messages enter and leave the mail flow components. You can create messaging policy and rule settings that are designed to meet different regulations and that can easily be changed to adapt to your organization's requirements. The transport-based messaging policy and compliance features include server-based rules that you configure to enforce your organization's compliance scenarios and the Journaling agent that acts to enforce message retention. For more information, see Planning for Hub Transport Features.

The Exchange 2007 Built-in Protection features provide anti-spam and antivirus protection for messages. Although these Built-in Protection features are designed for use in the perimeter network on the Edge Transport server role, the Edge Transport agents can also be configured on the Hub Transport server. By default, these agents are not enabled on the Hub Transport server role. To use the anti-spam features on the Hub Transport server, you must register the agents in a configuration file and enable the features that you want to use by running a provided Exchange Management Shell script. You install and enable the antivirus agent in a separate operation. For more information, see Planning for Anti-Spam and Antivirus Features.

For more information about how to plan for the Hub Transport server role and the features that this role provides, see the following topics:

• Planning for Hub Transport Servers
  
  
• Transport Architecture
  
  
• New Messaging Policy and Compliance Features
  
  
• New Anti-Spam and Antivirus Functionality