A Hybrid Exchange deployment consists of an on-premise Exchange Server installation working together with Exchange Online, which is also known as coexistence. As we know, Exchange Online is Microsoft’s hosted Exchange service for small businesses and enterprises, and is typically acquired as a bundle as part of the company’s Office 365 cloud offering.

In this post I will highlight some of the benefits, components and other considerations that need to be taken before going for a Hybrid Exchange deployment.

Benefits of a Hybrid Exchange Deployment

Despite the greater complexity, there are a number of tangible reasons why businesses may want to try a hybrid deployment. Services such as Exchange Online Archiving (EOA) can be used with a hybrid deployment, which may appeal to smaller businesses due to its low-barrier access to 99.9% uptime, 24/7 live phone support and ability for users to access their own email archives from their Inbox.

In some cases, organizations can reduce cost by relocating users on inexpensive cloud servers but with important data hosted on-premise. This can be crucial in terms of legal jurisdiction of data for foreign companies not headquartered in the U.S. From a technical perspective, Exchange administrators can also run PowerShell commands that are mirrored to online servers, while a single Outlook Web App URL is used to access both on-premise and cloud-based Exchange deployment.

Do note that while it is possible to set up Exchange for coexistence with another system such as Lotus Notes, the term “hybrid mode” here has specific connotations that pertain specifically to an on-premise Exchange 2010 working with Exchange Online.

Components required for a hybrid deployment

An organization that wants to set up a Hybrid deployment must first configure the cloud-based portion of the Exchange deployment. Furthermore, an Exchange 2010 server with SP1 in the CAS server role is required to be installed on-premise to enable a hybrid deployment. This includes an extension of the Active Directory schema when installing Exchange 2010, as well as the setting up of a DirSync server in order to maintain a synchronized address book between the cloud and on-premise installation.

The Dirsync server will need to be appropriately sized to meet hardware requirements; a 1.6GHz CPU with 4GB memory and 70GB of hard disk space is adequate to support an Active Directory with up to 50,000 objects. Note that a ticket with support will need to be opened for organizations with more than 20,000 Active Directory objects. Also, an Exchange hybrid deployment is not supported for an organization that implements multiple forests for logon or resource segmentation.

Offboarding or decommissioning of on-premise Exchange Server

Aside from the benefits outlined earlier, another use of a hybrid deployment is as an intermediate step to decommissioning an on-premise Exchange so as to switch completely to Exchange Online. In addition, it can also facilitate an organization that started with Exchange Online but decided to move mailboxes back on-premises, in a move called offboarding.

For the former, Microsoft advices that the steps leading to the decommissioning of an on-premise Exchange deployment should be planned carefully with the help of an Office 365 deployment specialist. For moving mailboxes from Exchange Online to an on-premise deployment, you can check out Exchange Hybrid Deployment – Moving Cloud-Based Mailboxes to the On-Premises Organization for more information.

While we have barely scratched the surface of Hybrid Exchange deployment, you should now have a better understanding of what considerations and benefits it involves.


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