Linked Mailboxes

Linked mailboxes are used in organizations where the Exchange server is deployed in a separate forest called resource forest. User accounts that are located in other trusted forests are called account forests. The user accounts are allowed to access their linked mailboxes in the resource forest. User accounts don't exist in the forest where Exchange is deployed. Therefore, disabled user accounts are created in the resource forest and associated with the corresponding linked mailbox.

For more details, please see the Manage linked mailboxes in Exchange Server article.

Exchange Resource Forest Model

The exchange resource forest model should satisfy the following conditions:

There are two separate forests in the environment: Exchange resource forest and account forest.

Exchange resource forest:

• It is used only for resources.

• It doesn't contain any user accounts, it contains service accounts and Exchange resource forest administration accounts only.

• Microsoft Exchange server is installed.

Account forest:

• All user accounts should be created in a separate account forest.

• At least a one-way outgoing trust should be created so that the Exchange Resource forest trusts the account forest.

For more details, review the following Microsoft article: https://learn.microsoft.com/en-us/exchange/deploy-exchange-2013-in-an-exchange-resource-forest-topology-exchange-2013-help