In Exchange 2010 SP1, you can use Exchange Online Archiving (EOA) to provision a cloud-based archive for your on-premises mailbox users. To use EOA, you must sign up for EOA service from Exchange Online. Additionally, you must follow the steps required to set up your organization for EOA. These steps include configuring Active Directory synchronization, and installing and configuring a hybrid server.
Continue at source.
Technology Architect/Writer/MS Vendor
MCM: Exchange Server | MVP: Exchange Architecture