Microsoft

Outlook and Hotmail users can blame the recent outage on an overheated data center, Microsoft says.

On Tuesday at around 1:30 p.m. PT, the two online e-mail services suffered a service disruption, rendering them inaccessible to many users. Microsoft started to bring them back online the rest of the day and on into Wednesday. But access wasn't fully restored until 5:43 a.m. yesterday, according to the company.

Microsoft's status page confirmed that the problem was repaired but offered no details as to the cause. Now the company has revealed the source of the glitch in a blog from Arthur De Haan, a vice president of test and service engineering for Windows Services.

On Tuesday, Microsoft updated firmware in a data center that stores certain parts of Hotmail, Outlook, and SkyDrive. This update is performed on a regular basis and usually runs smoothly. This time it failed, triggering a rapid and huge rise in temperature in that data center.

The spike in heat caused certain safeguards to kick in for many of the servers in that location. Those safeguards apparently cut off access to Hotmail and Outlook mailboxes and prevented any automatic failover that would've kept the services up and running. After the safeguards came on, a Microsoft team stepped in to try to restore access.

Why did the problem take so long to fix?

"Based on the failure scenario, there was a mix of infrastructure software and human intervention that was needed to bring the core infrastructure back online," De Haan said. "Requiring this kind of human intervention is not the norm for our services and added significant time to the restoration."

De Haan also said that Microsoft is working hard to make sure this type of problem doesn't happen again. But Outlook and Hotmail have been hit by other issues this year.

In January, mobile users of the two services were unable to access their e-mail for several days. In late February, Outlook.com was inaccessible to some users.

Microsoft is phasing out Hotmail in favor of Outlook and is in the process of moving Hotmail users to their new Outlook.com accounts.