With the previous version of the Exchange 2010 MP (14.03.0038.003) there was a known Issue with Mailboxes entering into a Quarantined State. I have posted some details below to help you identify Mailboxes that were Quarantined, Backup and Remove the Overrides and Reference Overrides, then remove the mailbox from Quarantine.
Event ID: 10018
Task Category: General
The mailbox for user dd723faa-dd63-4d02-8635-230fce615714: /o=DOMAIN/ou=First Administrative Group/cn=Recipients/cn=JUSER has been quarantined. Access to this mailbox will be restricted to administrative logons for the next 6 hours.
1. Export the Exchange 2010 Overrides and Reference Overrides
a. Open the Exchange Management Shell as an Exchange Administrator
b. Run the syntax of Get-MailboxStatistics -identity ‘username’ | flc. Note the Mailbox GUID
6. Restart the ‘Microsoft Information Store’ service
7. Clients are able to reconnect to Exchange