The hybrid server requires the following Exchange 2010 server roles to be installed:

• Client Access server role   The Client Access server role on the hybrid server provides the functionality typically provided by a front-end server in Exchange 2003. All client connectivity, including Outlook client access, Outlook Web App, and Outlook Anywhere goes through the Client Access server role. Organization relationship features between the on-premises and cloud-based organizations, such as free/busy sharing, are also handled by the Client Access server role.
  
  Learn more at: Understanding Client Access
  
  
• Hub Transport server role   The Hub Transport server role on the hybrid server handles all mail flow between the on-premises and cloud-based Exchange organizations and between the on-premises organization and the Internet. It helps to secure transport communication between the on-premises and cloud-based organizations, as well as handling transport rules, journaling policies, and message delivery to user mailboxes in a hybrid deployment.
  
  Learn more at: Overview of the Hub Transport Server Role
  
  
• Mailbox server role   The Mailbox server role on the hybrid server hosts replicas of the OU=EXTERNAL (FYDIBOHF25SPDLT) and OU=Exchange Administrative Group (FYDIBOHF23SPDLT) public folders that enables calendar free/busy information sharing between the on-premises and cloud-based organizations. Mailboxes should not be created on the hybrid server in a typical hybrid deployment between an on-premises Exchange 2003 organization and a cloud-based organization.
  
  Learn more at: Overview of the Mailbox Server Role
  
  

The hybrid server provides several important functions for your on-premises organization in a hybrid deployment:

• Federation   The hybrid server enables you to create a federation trust for your on-premises organization with the Microsoft Federation Gateway. The Microsoft Federation Gateway is a free, cloud-based service offered by Microsoft that acts as the trust broker between your on-premises organization and the cloud-based organization. Federation is a requirement for creating an organization relationship between the on-premises and the cloud-based organizations.
  
  Learn more at: Understanding Federation
  
  
• Organization relationships   The hybrid server enables you to create organization relationships between the on-premises and cloud-based organizations. Organization relationships are required for many other services in a hybrid deployment, including calendar free/busy information sharing, message tracking, and mailbox moves between the on-premises and cloud-based organizations.
  
  Learn more at: Understanding Federated Delegation
  
  
• Message transport   The hybrid server is responsible for message transport in a hybrid deployment. Using Send and Receive connectors, it replaces the on-premises Exchange 2003 server as the connection endpoint for incoming external messages and also provides outbound message delivery to the Internet and the cloud-based organization.
  
  Learn more at: Understanding Transport
  
  
• Message transport security   The hybrid server helps to secure message communication between the on-premises and cloud-based organizations by using the Domain Security functionality in Exchange 2010. Security can be increased by using mutual transport layer security authentication and encryption for message communications.
  
  Learn more at: Understanding Domain Security
  
  
• Outlook Web App   The hybrid server supports configuring a single URL endpoint for external connections to on-premises and cloud-based mailboxes. For on-premises mailboxes, the hybrid server can be configured to automatically redirect user Outlook Web App requests to your Exchange 2003 mailbox server. For cloud-based organization mailboxes, the hybrid server can be configured to automatically display a link to the Outlook Web App endpoint on the cloud-based organization.
  
  
• Learn more at: Understanding Outlook Web App
  
  

The hybrid server is deployed much like an Exchange 2010 server would be deployed to your existing Exchange 2003 organization. Using the Client Access, Mailbox, and Hub Transport server roles, the hybrid server is responsible for many services for your on-premises organization that are currently provided by your existing Exchange 2003 server. The following table describes briefly the changes in services after configuring a hybrid deployment.

Service Pack 1 (SP1) for Exchange Server 2010 provides the base for hybrid deployment functionality. You can use any Exchange 2010 SP1 media when installing the hybrid server.

Download Exchange Server 2010 SP1 at: Exchange 2010 Service Pack 1 (SP1)

Important:
You need to provide an Exchange 2010 Hybrid Edition product key on the hybrid server when you configure a hybrid deployment with Office 365. To obtain a Hybrid Edition product key, contact Office 365 support.