Exchange 2010 has a security measure when coming across a 'poisoned' mailbox in its Information Store database. If it deems the mailbox is 'dirty' or 'poisoned', it will quarantine the mailbox. The end result is that the end user is unable to access mail. The only way to access the mailbox and its contents is by having administrative access to the mailbox.
This can happen if a mailbox has too many requests by MailEssential's 'Move to Sub folder' action. Please change to action for the anti-spam filters to another option, such as 'Move to Junk Email Folder' and release the mailbox from the quarantine.
To release the mailbox and get mailbox accessible to the user, please follow the steps in this Microsoft article: