Quarantined mailboxes detected on Microsoft Exchange Server 2010

A user is not able to access a mailbox that resides on a Microsoft Exchange 2010 Server.

The mailbox is a potential threat to the health of the Information store and has been quarantined. 

There are several types of events for which the Exchange store tags a mailbox as a potential threat:

• If a thread doing work for that mailbox fails
• If there are more than five threads in that mailbox that haven't made progress for a long time
  

The cause of the poison mailbox must be identified and corrected. Once this is accomplished, to gain access to the mailbox immediatly, the registry key for the quarantined mailbox should be reset manually by deleting it. 

Note: If this manual step is forgotten, the Exchange store automatically resets quarantined mailboxes six hours after the quarantined flag was set.

For more information on this topic, see the following document from Microsoft TechNet online:

Poison Mailbox Detection and Correction
http://technet.microsoft.com/library/bb331958.aspx#SH