“Although you can synchronize free/busy information across forests and use it to schedule meetings, you cannot use the Open Other User's Folder feature in Microsoft Office Outlook to view the calendar details for a user in another forest”
----------Synchronizing Multiple Exchange 2003 Forests
For opening the calendar in another forest, the following is the options I know:
· Exchange 2010’s new feature, Federation can easily share the calendar cross the forests
· You can use the linked mailbox to open the calendar in another forest
· If you are using outlook 2007, the calendar can be published to internet, and then share to person in another company, but with read-only permission
Resources:
Deploy Exchange 2010 in an Exchange Resource Forest Topology
Internet Calendars for Microsoft Office Outlook 2007
James Luo
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