Edited: November 11, 2015
In a hybrid environment the on-premises Exchange organization (which can be either Exchange 2010 or Exchange 2013) is integrated with Exchange Online. In a hybrid configuration you basically create one ‘virtual’ Exchange organization with the following features:
- One cross-premises Address Book;
- Secure cross-premises mail flow;
- Cross-premises Free/Busy information, mail tips and out-of-office features;
- Seamless migration to Exchange Online and vice versa;
- No recreation of OST file;
- Automatic reconfiguration of Outlook profile;
- OWA URL Redirect.
To create a Hybrid environment you need at least one Exchange hybrid server on-premises. This can be an Exchange 2010 server but I always recommend using an Exchange 2013 server for this because of the improved hybrid connectivity in Exchange 2013. For redundancy purposes (and performance for larger environments) you better use multiple Exchange 2013 Hybrid servers.
Another prerequisite for creating a Hybrid environment is that you must have Directory Synchronization in place, so DirSync is used for synchronization user accounts, groups and contacts, all other communication is handled by the Exchange 2013 hybrid servers as shown in the following picture: