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 10018 in the Exchange CAS Application Log:
Log Name:      Application
Source:        MSExchangeIS
Event ID:      10018
Task Category: General
Level:         Error
Keywords:      Classic
User:          N/A
Computer:      SVRCAS01.DOMAIN.COM
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.


Steps taken:
1.      Export the Exchange 2010 Overrides and Reference Overrides
2.      Remove the Exchange 2010 Overrides and Reference Overrides
3.      Remove the Exchange 2010 MP
4.      Check for all Quarantined Mailboxes
a.      Open the Exchange Management Shell as an Exchange Administrator
b.      Run the syntax of Get-MailboxStatistics -identity ‘username’ | fl
c.      Note the Mailbox GUID
Note: The database hosting the mailbox needs to be remounted, or the Exchange store restarted, for the reset of the quarantined mailbox to take effect.  You can also wait up to 6 hours for Exchange to Automatically remove the Quarantined Mailbox.

6.      Restart the ‘Microsoft Information Store’ service

7.      Clients are able to reconnect to Exchange