This error occurs under the following conditoins:
- The mailbox is configured to use Server-Side Synchronization for Appointments, contacts, and tasks.
- The associated Exchange mailbox is already configured with another Dynamics 365 organization.
A user can be a member of more than one Dynamics 365 organization, but an Exchange mailbox (email address) can only synchronize appointments, contacts, and tasks with one organization, and a user that belongs to that organization can only synchronize appointments, contacts, and tasks with one Exchange mailbox. Dynamics 365 stores the organization ID (OrgID) for the synchronizing organization and the last time the user synced in Exchange.
For example: Your user record may exist in a Dynamics 365 organization used for production usage and your company may also have another organization used for something else such as training, testing, or development. Your Exchange mailbox can only be configured to use Server-Side Synchronization with one of the organizations.