A few users had reported Outlook 2003 Shared Calendar issues when migrating to Exchange 2010. Although Outlook 2003 is “Supported” on Exchange 2010 there are a few issues. The most annoying one I came across was the inability to open more than about 5 shared calendars or mailboxes. Users get a misleading error “The connection to the Microsoft Exchange Server is unavailable. Outlook must be online or connected to complete this action.”
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