We wanted to bring to your attention an issue that we have been seeing recently in CSS where many mailboxes on the same mailbox database become quarantined seemingly for no reason. Upon inspecting the Application log, you will see many 10018 events with text containing to the following information.
Log Name: Application
Event ID: 10018
Task Category: General
The mailbox for user <guid>: /o=Contoso /ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=UserMailbox has been quarantined. Access to this mailbox will be restricted to administrative logons for the next 6 hours.
Read more at source: http://blogs.technet.com/b/exchange/archive/2012/06/28/mailboxes-on-a-database-are-quarantined-in-an-environment-with-system-center-operations-manager.aspx