For customers wanting to take advantage of the hybrid model for migrations or coexistence, Microsoft is now offering a Hybrid Server license free of charge. The Hybrid server is not a custom version of Exchange, but one with CAS and Hub roles for Exchange 2010 and CAS and Mailbox roles for Exchange 2013. Mailboxes are not allowed to be hosted on this server and its only role is as a middle man in a hybrid deployment to Office 365.
This sever role also adds some new options for migration strategies to Office 365. If the on premise deployment is Exchange 2003 then an Exchange 2010 SP3 server is required for Hybrid. If the deployment is Exchange 2007+ then an Exchange 2013 SP1 is recommended as there are updated feature sets available.
A license key from Microsoft can be obtained at:
