Symptoms
Microsoft SharePoint 2010 calendar has the Calendars Overlay feature that lets the calendar retrieve calendar items from a Microsoft Exchange server. However, the Calendars Overlay feature works only with Exchange servers in the same forest as the SharePoint 2010 server. If you try to retrieve calendar items from an Exchange server that is in a different forest, the operation is unsuccessful.
Status
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
