Just installed Exchange Server 2010 RC and when trying to test it with my outlook in IMAP configuration I received this error:
"Task 'Synchronizing subscribed folders for firstname.lastname@example.org.' reported error (0x8004DF0B) : 'Outlook cannot synchronize subscribed folders for email@example.com. Error: The connection to the server is unavailable. Outlook must be online or connected to complete this action. If you continue to receive this message, contact your server administrator or Internet service provider (ISP).'"
The problem was that for some reason after installing Exchange Server 2010 RC, service Microsoft Exchange IMAP4 did not start automatically, so I had to open services.msc (via "run"), and start this service manually, also changed its startup type to Automatic for future. This solved the problem.