Email Services - Frequently Asked Questions:
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Why didn't my meeting sync to all of my clients, such as Outlook, OWA, and my mobile device?
If you accepted the meeting from a Mac using Outlook 2011, and did not send a response, Outlook only updates the local Mac calendar and doesn't sync the meeting to the server. To be sure a meeting is synced to all clients, you must send a response when accepting meeting requests with Outlook 2011.
Microsoft has more information about this, though in our experience the issue only occurs when the meeting requests originates from an outside service.
Information Systems and Computing, University of Pennsylvania