The company said the leak is the result of Windows Phone 7's Exchange Activesync software's failure to synchronize properly with Yahoo Mail. "This inefficiency can result in larger than expected data usage for a very small percentage of users," Microsoft said in a statement.
The admission comes after thousands of Windows Phone 7 customers reported maxing out their network data allowances despite only moderate usage. Initially, it was believed that a feature on the phone that allows Microsoft to monitor performance was the culprit.
But Microsoft said it has now definitely identified the source of the leak as a faulty connection between EAS and Yahoo Mail. For customers who use that service, the company recommends they synchronize their mail less frequently.
Microsoft also warned that the bug in EAS could also create data loss problems for users of other e-mail services, including Google's Gmail and even its own Outlook. Microsoft said it's working on a patch to address the issue, but did not provide a date on which the fix would be available for download.
Microsoft released Windows Phone 7 in November, with an eye to making a dent in Apple and Google's dominance of the smartphone market. Dell, HTC, and Samsung offer versions of the devices, while AT&T and T-Mobile are the exclusive carriers in the U.S.
Windows Phone 7 devices, regardless of manufacturer, deliver a common user experience based on an interface Microsoft calls Live Tiles. The tiles allow users to quickly access calling, social media, messaging, photo, e-mail, and personal applications and services. They also deliver real-time information to the main interface.
Microsoft has said 1.5 million Windows Phone 7 handsets were shipped to retailers during the first six weeks of availability. The tally, however, does not represent the number of units actually sold.
Gartner predicts the release of Windows Phone 7 will help bump Microsoft's share of the worldwide mobile OS market from 4.7% in 2010 to 5.2% in 2011, but says the company's share will ultimately decline to just 3.9% by 2014.