Windows 7 and Windows Phone 7 expert Paul Thurrott collected reports in early January that supported his own observation of mysterious large amounts of data (30 to 50MB per day) being uploaded from Windows Phone devices over 3G even when connected to a WiFi network. A quick check of my own billing did not indicate my HTC HD7 Windows Phone exhibited this problem.

Windows Phone Mystery Data Upload Problem Only on AT&T?

Later (Jan. 20), Microsoft confirmed that a “third party solution” was the root of this mystery data upload. However, the third party was not named and no app (or something else) was identified.

Microsoft Blames Unnamed Third Party for Mysterious 3G Data Uploads

Yesterday, Thurrott confirmed that the Yahoo! mail component provided by Microsoft for Windows Phone was the source of the data leak based on investigative work by Thurrott’s Windows 7 Secrets book co-author Rafael Rivera.

Now Microsoft confirms Yahoo! as the data leak culprit

A workaround that can be used until a fix is available is also provided in the article. The trick? Set the frequency to download new content and email to longer time periods.