A hybrid deployment enables the following features:

• Secure mail routing between on-premises and Exchange Online organizations.
  
  
• Mail routing with a shared domain namespace. For example, both on-premises and cloud-based organizations use the @contoso.com SMTP domain.
  
  
• A unified global address list, also called a “shared address book”.
  
  
• Free/busy and calendar sharing between on-premises and Exchange Online organizations.
  
  
• Centralized control of outbound mail flow. You can configure Exchange Online to route all messages to Internet recipients through the on-premises Exchange organization.
  
  
• A single Outlook Web App URL for both the on-premises and Exchange Online organizations.
  
  
• The ability to move existing on-premises mailboxes to the Exchange Online organization.
  
  
• Centralized mailbox management using the on-premises Exchange Management Console.
  
  
• Message tracking, MailTips, and multi-mailbox search between on-premises and Exchange Online organizations.
  
  
• Cloud-based message archiving for on-premises Exchange mailboxes. Exchange Online Archiving can be used with a hybrid deployment. For more information, see Understanding Exchange Online Archiving.
  
  

A hybrid deployment involves several different services and components:

• Microsoft Office 365   Office 365 provides a cloud-based Exchange Online organization as a part of its subscription service. Organizations configuring a hybrid deployment must create and configure this cloud-based Exchange organization.
  
  
• Hybrid Configuration wizard   Service Pack 3 (SP3) for Exchange Server 2010 includes Hybrid Configuration wizards which provide you with a streamlined process to configure a hybrid deployment between on-premises Exchange and Exchange Online organizations.
  
  Learn more at: Understanding the Hybrid Configuration Wizard
  
  
• Hybrid server   When the Hybrid Configuration wizards complete the configuration of an Exchange 2010 SP3 server in your existing Exchange organization, that Exchange 2010 SP3 server is now referred to as a hybrid server. Hybrid servers are required for hybrid deployments. They enable messaging features and message delivery between your on-premises Exchange and Exchange Online organizations.
  
  
• 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 Exchange 2010 organization and the Exchange Online organization.
  
  On-premises organizations configuring a hybrid deployment must have a federation trust with the Microsoft Federation Gateway. The Hybrid Configuration wizard checks to see if there is an existing federation trust with the Microsoft Federation Gateway for the on-premises organization. If present, the existing federation trust is used to support the hybrid deployment. If not present, the wizard creates a federation trust for the on-premises organization with the Microsoft Federation Gateway. A federation trust with the Microsoft Federation Gateway for your Office 365 tenant is automatically configured when you activate your Office 365 service account.
  
  Learn more at: Microsoft Federation Gateway
  
  
• Active Directory synchronization   Active Directory synchronization replicates on-premises Active Directory information for mail-enabled objects to the Office 365 organization to support the unified global address list (GAL). Organizations configuring a hybrid deployment must deploy Active Directory synchronization on a separate on-premises server.
  
  Learn more at: Active Directory synchronization: Roadmap