Exchange 2010 will quarantine a corrupt, or poisoned, mailbox in the Information Store database. If a mailbox is corrupt, dirty or poisoned, rather than forcing us to run eseutil or isinteg while the database is offline, Exchange just quarantines the mailbox. If you run into one of these, you can remove from the quarantine to run a mailbox repair by deleting a registry key. To figure out which key to run, first locate the GUID of the mailbox using PowerShell:
Get-MailboxStatistics -identity USERNAME | fl
Then copy the mailbox GUID and open up the registry and make a backup (which I do every time I change the registry btw) of the registry. Then view the following key:
Delete the key for the mailbox that displays as poisoned. Then, restart the Information Store and run a quick iisreset.