In order for Enterprise Vault to be able to archive mailboxes on an Exchange Server in another Windows Server forest, a forest trust is required. The forest trust is only between the forest root domains. A trust is defined as a connection between two domains used for authentication. Trusts are automatically established between domains within the same forest.
Note: Ensure that Domain Name System (DNS) is properly set up.
Company A purchases Company B and has merged both forests into one. Company A requires to target Company B’s Exchange Server and archive mailboxes with Enterprise Vault.
Note: Forest trusts can only be created between two forests and cannot be implicitly extended to a third forest. This means that if a forest trust is created between forest 1 and forest 2, and a forest trust is also created between forest 2 and forest 3, forest 1 will not have an implicit trust with forest 3.
Enterprise Vault and User Objects reside in Domain A, Exchange Server resides in Domain B. In this configuration with the Users Objects and the Enterprise Vault server resides in Domain A, and the Exchange server resides in a separate domain. This scenario requires only a One-way trust from Domain B to Domain A, it will allow the Exchange server to Authenticate the Service accounts.
For additional information on trusts please refer to the following article: