Here is a quick overview of the Exchange 2010 server roles in a hybrid deployment:

• Client Access server role   The Client Access server role continues to provide essentially the same functionality typically provided by Client Access servers in your Exchange 2010 organization with a few additions required to support a hybrid deployment. 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 Exchange Online 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 handles all mail flow between the on-premises and Exchange Online organizations and between the on-premises organization and the Internet. It helps to secure transport communication between the on-premises and Exchange Online 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
  
  

Depending on the hybrid deployment configuration that you want, an Exchange 2010 SP3 server requires one or more of the server roles to be installed on it:

• Single Exchange server   If you choose to install a single Exchange server in your on-premises organization, you’ll need to install the Mailbox, Client Access, and Hub Transport server roles on the single server.
  
  
• More than one Exchange server   If you choose to install more than one Exchange server in your on-premises organization, you can install the server roles on separate servers in your on-premises organization. For example, you could install one Exchange server that has the Mailbox and Client Access roles installed and also install another Exchange server that has only the Hub Transport server role installed. However, the best practice and recommended server configuration is to install the Client Access and Hub Transport servers on each server deployed in your on-premises organization.
  
  

If you also decide to install the optional Exchange 2010 Mailbox server role in your hybrid deployment, you should add the Mailbox server role to each Exchange server that has the Client Access and Hub Transport server roles installed. Learn more about the Mailbox server role at Overview of the Mailbox Server Role and learn more about Exchange capacity planning at Understanding Multiple Server Role Configurations in Capacity Planning.

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

• Federation   Exchange servers enable 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 Office 365 tenant organization. Federation is a requirement for creating an organization relationship between the on-premises and the Exchange Online organizations.
  
  Learn more at: Understanding Federation
  
  
• Organization relationships   Exchange servers with the Client Access server role enable the creation of organization relationships between the on-premises and Exchange Online 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 Exchange Online organizations.
  
  Learn more at: Understanding Federated Delegation
  
  
• Message transport   Exchange servers with the Hub Transport server role are responsible for message transport in a hybrid deployment. Using Send and Receive connectors, they serve as the connection endpoint for incoming external messages and also provide outbound message delivery to the Internet and the Exchange Online organization.
  
  Learn more at: Understanding Transport
  
  
• Message transport security   Exchange servers with the Hub Transport server role help to secure message communication between the on-premises and Exchange Online 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   Exchange servers with the Client Access server role support configuring a single URL endpoint for external connections to on-premises and Exchange Online mailboxes. For on-premises mailboxes, Client Access servers are configured to service Outlook Web App requests.. For Exchange Online organization mailboxes, Client Access servers are configured to automatically display a link to the Outlook Web App endpoint on the Exchange Online organization.
  
  Learn more at: Understanding Outlook Web App
  
  

If you choose to add additional Exchange servers to support your hybrid deployment, the Exchange server would be deployed much like any other Exchange 2010 server would be deployed to your existing Exchange 2010 organization. Configuring your existing on-premises Exchange 2010 organization for a hybrid deployment doesn’t require any special Exchange server topology. The following table describes briefly the changes in services after configuring a hybrid deployment.

Exchange 2010 SP3 enables hybrid deployment functionality with the Hybrid Configuration wizards. You can use any Exchange 2010 SP3 media when installing additional Exchange 2010 servers.

Additionally, we recommend installing future Update Rollups 4 for Exchange 2010 SP3 on all your hybrid servers. Microsoft releases update rollup packages approximately every six to eight weeks. The rollup packages are available via Microsoft Update and the Microsoft Download Center. In the Search box on the Microsoft Download Center, type "Exchange 2010 SP3 update rollup" to find links to the rollup packages for Exchange 2010 SP3.

Download Exchange Server 2010 SP3 at: Exchange 2010 Service Pack 3 (SP3)

Find update rollup packages at: Microsoft Download Center

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.