we do want to sincerely apologize to anyone that was unable to access their email during the interruption. Outages are something we take very seriously and invest a significant amount of our time and energy in doing our best to prevent. Root cause analysis
At 13:35 PM PDT on March 12th, 2013 there was a service interruption that affected some people's access to a small part of the SkyDrive service, but primarily Hotmail.com and Outlook.com. Availability was restored over the course of the afternoon and evening, and fully restored by 5:43 AM PDT on March 13th, 2013.
On the afternoon of the 12th, in one physical region of one of our datacenters, we performed our regular process of updating the firmware on a core part of our physical plant. This is an update that had been done successfully previously, but failed in this specific instance in an unexpected way. This failure resulted in a rapid and substantial temperature spike in the datacenter. This spike was significant enough before it was mitigated that it caused our safeguards to come in to place for a large number of servers in this part of the datacenter.
These safeguards prevented access to mailboxes housed on these servers and also prevented any other pieces of our infrastructure to automatically failover and allow continued access. This area of the datacenter houses parts of the Hotmail.com, Outlook.com, and SkyDrive infrastructure, and so some people trying to access those services were impacted.Microsoft Explains Outlook and SkyDrive Outage [blogs.office.com]