Organizations often need to share availability (aka Free/Busy) information with other Exchange organizations. Depending on the version of Exchange Server you're on, there are different ways to do this. Let's take a look at three common scenarios and go through the exact steps required in order to achieve sharing of availability information between two on-premises Exchange organizations.
Read the complete article at http://blogs.technet.com/b/exchange/archive/2011/06/28/cross-org-availability-using-federation-trust-and-organization-relationship.aspx
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