Over the next several months Microsoft will deploy the Exchange Online December 2011 Service Update. As a part of this update, we will also make high availability architecture enhancements in all datacenters that host Office 365 tenant domains. These updates will be staggered globally, beginning in December 2011, and we expect full deployment to complete within six months. Approximately one week before the service update is deployed to your region we will post more information in the Planned Maintenance section in your Service Health Dashboard portal. Please check this resource for awareness of when your update is scheduled to begin.
Read the complete blog at http://blogs.technet.com/b/exchange/archive/2011/12/07/exchange-online-december-2011-service-update.aspx
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