“The Outlook.com partial outage lasting 16 hours on Tuesday and Wednesday morning was caused by a firmware update gone awry that triggered a temperature spike in a Microsoft data center, resulting in automatic safeguards that made a large number of servers inaccessible.”
( http://catless.ncl.ac.uk/Risks/27.20.html#subj14 )
See also http://www.theregister.co.uk/2013/03/14/microsoft_hot_servers/, and the report referenced by the RISKS post at http://www.networkworld.com/news/2013/031413-microsoft-outlook-outage-267705.html
http://blogs.office.com/b/microsoft-outlook/archive/2013/03/13/details-of-the-hotmail-outlook-com-outage-on-march-12.aspx