Hi,@Nicholas
Thanks for posting your question in the Microsoft Q&A forum.
Based on the information you've provided, I'd like to confirm a few things with you:
1.Do you create a two-way trust between the source and target forests to ensure that users in both forests can authenticate and authorize resources in the other forest? If not you can refer to this link:https://learn.microsoft.com/en-us/entra/identity/domain-services/tutorial-create-forest-trust
2.Do you just want to get the free/busy status of users in another forest? If so, you can configure federated sharing.https://learn.microsoft.com/en-us/exchange/configuring-federated-sharing-between-exchange-organizations-exchange-2013-help?source=recommendations
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