Exchange Hybrid is really a rich coexistence option provided by Microsoft, by leveraging existing native Exchange tools. Exchange Hybrid offers customers with an existing Exchange 2003, 2007, or 2010 environment the capability to integrate directly your Office 365 subscription. Think of the integration as merely an extension of your environment. The integration components can be enabled, when a customer deploys Exchange 2010 (SP1+) in their environment. Integration is established by creating an Exchange Federation agreement from the on-premise Exchange Organization, to the Office 365 subscription. The benefits of Exchange Hybrid are:
Mail routing through Exchange 2010, leveraging TLS
Mail routing control (mail flow from/to Office 365 or on-premise)
Maintaining a long-term Exchange mailbox presence on-premise and in Office 365
Calendar Free/Busy lookup between Exchange on-premise and Office 365 mailboxes...