Shared free/busy in Exchange hybrid deployments

Sharing free/busy (also known as calendar availability) information between users in on-premises Exchange and Exchange Online is one of the primary benefits of a hybrid deployment. Users in both organizations can view each other's calendars as if they were located in the same physical organization. This configuration makes scheduling meetings and resources easy and efficient.

Several components in a hybrid deployment are required to enable the shared free/busy feature between your on-premises Exchange organization and Exchange Online.

  • Federation trust (For mixed Exchange 2013/2010 and Exchange 2013/2007 hybrid deployments only): Both the on-premises and clouds organizations need an established federation trust with the Microsoft Entra ID. A federation trust is a one-to-one relationship with the Microsoft Entra authentication system that defines parameters for your Exchange organization. The system uses these parameters and acts as a trust broker to exchange free/busy information between on-premises and Exchange Online organization users.

    A federation trust s automatically configured for your Microsoft 365 service when the account is created. The Hybrid Configuration wizard automatically checks for an existing federation trust between Microsoft Entra ID and the on-premises organization. If present, the existing federation trust is used to support the hybrid deployment. If not, the wizard creates a federation trust for the on-premises organization with Microsoft Entra ID. The wizard also adds any domains within the Hybrid Configuration wizard to the on-premises organization federation trust.

    Learn more at Sharing.

  • Organization relationships: Organization relationships are needed for both the on-premises Exchange and Exchange Online organization. These relationships are configured automatically by the Hybrid Configuration wizard. An organization relationship defines the level of free/busy information that's shared from the organization.

    By default, the free/busy data access sharing level is Free/busy access with time, plus subject and location for both the on-premises and Exchange Online organization relationships. You can manually configure the organization relationship access level after the Hybrid Configuration wizard has completed.

    Learn more at Sharing.

Shared free/busy calendar access is automatically configured by the Hybrid Configuration wizard in all scenarios. A federation trust with Microsoft Entra ID and organization relationships for on-premises Exchange and Exchange Online are hybrid deployment requirements. If you don't want to allow free/busy sharing between your on-premises and Exchange Online users, you can manually disable it by using the Set-HybridConfiguration cmdlet after the Hybrid Configuration wizard has completed.

Learn more at Hybrid components.

The hybrid deployment features shown in the following table have a dependency on federation trusts and organization relationships.

Messaging area Feature
Email client Message tracking


Multi-mailbox search

Compliance Exchange Online Archiving

Exchange In-place eDiscovery