We had a problem with a couple of users who kept having synchronisation problems with an Exchange (2010) server, using Outlook 2010. Some mail would not appear in mailboxes, but would appear in the web client. The sync would claim to complete ok, but looking at the Sync Issues folder there were a lot of messages of the form:
13:22:28 Error Synchronizing Hierarchy for Mailbox 'TeamShared' 13:22:28 [80040305-54A-4DE-1900] 13:22:28 Your server administrator has limited the number of items you can open simultaneously. Try closing messages you have opened or removing attachments and images from unsent messages you are composing.
This can occur if the secondary mailbox has a a lot of folders, which this one does – there’s a big tree of stuff in there. To solve this we turned off caching of the secondary mailbox. The details of the problem and the various workarounds are given at:
Performance problems when you try to access folders in a secondary mailbox in Outlook
