We’ve been migrating to hosted exchange on Office 365 for some weeks now. Some of our Outlook 2010 clients have a problem once they’d been moved.
After many hours of trying different things and searching I found a Microsoft Patch that resolved the problem.
The KB Number is – KB2791026 downloads can be found here KB2791026
Also check out my previous post about delegated mailboxes, you may find the two issues are linked