Assume that you configure the cross-forest Availability service between two forests that have Microsoft Exchange Server 2010 installed. In this situation, users who use Microsoft Office Outlook or Outlook Web Access (OWA) in the source forest cannot view the free/busy information of the mailboxes in the target forest.
This issue occurs because the Client Access server in the source forest uses the internal URLs of the Availability service in the target forest. When a user requests free/busy information, the Client Access server in the source forest uses the Autodiscover service to determine which URL of the Availability service in the target forest to query free/busy information. The Autodiscover service retrieves both the external URLs and the internal URLs that are configured for Exchange Web Services in the target forest. However, the source Client Access server only uses the internal URLs of the Availability service. Because the internal URLs of the target forest cannot be resolved by DNS, the request fails.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about how to configure the Availability service for cross-forest topologies, visit the following Microsoft website:
2545080 Users in a source forest cannot view the free/busy information of mailboxes in a target forest when the cross-forest Availability service is configured between two Exchange Server 2007 forests