If your on-premises Exchange organization is running Exchange 2007 or Exchange 2003, and if you want to connect your organization to Office 365 and your Exchange Online organization, you must install at least one on-premises Exchange 2013 or Exchange 2010 Service Pack 3 (SP3) server. This server is used for hybrid deployment connectivity that seamlessly connects your on-premises Exchange and Exchange Online organizations.
To avoid the additional cost of an Exchange 2013 or Exchange 2010 SP3 server license, you may qualify for a free Hybrid Edition product key for Exchange 2013 or Exchange 2010 SP3.
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