Microsoft has apologized, but not explained why nearly 20,000 Hotmail accounts were mysteriously emptied of their contents during the Christmas holiday.

Corporate vice president for Windows Live Chris Jones blogged on Monday that 17,355 Windows Live Hotmail accounts had lost all their email messages during the course of what he called "mailbox load balancing between servers."

Inboxes and folders starting emptying on December 30, with accounts appearing to be new and people receiving a "Welcome to Hotmail" email from Microsoft. Some affected accounts went back 10 years.

Users took to Hotmail forums pleading for Microsoft to restore their cherished accounts while other took to Facebook, launching a group to share their anguish and frustration with world+dog.

Jones responded on Monday to say that Microsoft had identified the problem by the evening of January 2 and that it had restored accounts – two days after messages went AWOL. He continued that Microsoft was sorry for the inconvenience to customers and partners.

"As with all incidents like this, we will fully investigate the cause and will take steps to prevent this from happening again," Jones said, without explaining how or why something as rudimentary as server load balancing should cause data to simply disappear.

The outage comes after a year in which Microsoft has relaunched and rebranded Hotmail with a new-look inbox intended to make it easier to manage and view emails, along with pictures, video, and links that might be sent along with the actual email messages.

The company in December asked Reddit readers to give their feedback on the changes. Hotmail remains the web's most popular free email service with 360 million users, but during the last decade it lost momentum to Google's Gmail in terms of new features and popularity.

Further, Microsoft has been PR-ing people hard, trying to convince us that Hotmail is just one of a suite of services that the company can competently and reliably deliver in – where else? – the cloud. ®