We've had some questions come through to the Exchange Server and Office 365 teams regarding which versions of Exchange Online can be configured to work in a hybrid environment (where you have some mailboxes online and some in the cloud) with Exchange Server on premises. For example, does Exchange Server 2013 on premises work with the currently available version of Exchange Online? See the following table:

(See The New Exchange Reaches RTM! for more information regarding the new Exchange Server.)

On-premises version Cloud tenant version  Status
Exchange 2010 SP1 Current Supported
Exchange 2010 SP2 Current Supported
Exchange 2010 SP3 Current Supported
Exchange Server 2013 Current Not supported
Exchange 2010 SP1 Forthcoming Not supported
Exchange 2010 SP2 Forthcoming Not supported
Exchange 2010 SP3 Forthcoming Supported
Exchange 2013 Forthcoming Supported