I recently ran into a problem with and Exchange 2010 server. I had a failure that caused Exchange to go down. I resolved the issue that caused the failure and was able to remount my databases. After mounting and testing I found that most of my users could not connect. When connecting to OWA the user would be presented with an error that stated that an unexpected error occurred.
This was a very frustrating problem because I could not find any information on the server side that would indicate the problem. There was no indication in the interface that would explain why the users were not able to access the system.
When trying to move one of these mailboxes I received an error that indicated that the mailbox was quarantined. Searching on this resulted in the article below. It turns out that when Exchange has a difficult time accessing a mailbox it will quarantine it forever. I cannot explain how irritating this was. The fix is to remove the quarantine reg keys for each affected mailbox and then remounting the database.
Backup your registry before making any changes. I make no promises or warranties on any actions taken to resolve this issue. If you do not feel comfortable I recommend contacting Microsoft for support.
Location of the RegKeys:
HKLM\SYSTEM\CurrentControlSet\Services\MSExchangeIS\<Server Name>\Private-{db guid}\QuarantinedMailboxes\{mailbox guid}
If you see mailbox guid reg key under the QuarantinedMailboxes key then you are experiencing this problem.
http://technet.microsoft.com/en-us/library/bb331958.aspx