Microsoft has admitted a problem between its Windows Phone Mail client and Yahoo Mail has caused the excessive Windows Phone 7 (WP7) data charges.
Microsoft previously blamed a "third-party solution" after receiving complaints from users about unidentified data use charges on WP7 devices. Some users were wrongly charged for up to 50MB a day for checking e-mail and social networking sites.
In a statement, Microsoft said it had determined an "inefficiency in the synchronisation of email between the Windows Phone Mail client and Yahoo Mail". A fix will be rolled out in the coming weeks.
The company also identified an issue with its Exchange ActiveSync [EAS] email synchronisation protocol, which affects Microsoft Outlook and Google Gmail. This causes mail to get stuck in the outbook if a user exceeds the mail server email size. An update will be issued in a near-term end user update.
Microsoft confirmed a "small percentage" of Windows Phone customers had experienced larger than expected data usage.
Blogger and programmer Rafael Rivera discovered Yahoo as the culprit of the data leak.
Microsoft launched Windows Phone 7 in October last year for nine different handsets. The company recently admitted it was only "comfortable" with WP7's progress after phone manufacturer sales reached 1.5 million.
According to the latest Canalys report, Microsoft's marketshare in the mobile market fell 20% from 7.2% in the fourth quarter of 2009 to 3.1% in the same period in 2010.
Read Computer Weekly's blog for a workaround for using Yahoo Mail on Windows Phone 7 devices.