Exchange 2010 SP1 and Exchange Online (Office 365) incorporate federation to enable secure organization-to-organization free/busy and calendar sharing with other Exchange organizations. Also new is the ability for users to publish their calendar in iCal format to anyone, and for them to subscribe to internet calendars. With Exchange now providing options for how your users can share calendar data with others inside and outside your organization, we've received a number of common questions about the differences in setup and functionality between sharing methods. These questions and their responses below can provide clarity as you consider calendar sharing in your organization.
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