Microsoft says it's discovered why some Windows Phone 7 phones have been blasting out large amounts of data without their owners' knowledge.
Over the last few weeks, users have been complaining that their phones have been making sneaky data transfers during the night, often pushing them over their monthly allowances.
Microsoft promised to investigate. And now - only after blogger Rafael Rivera worked it out himself - the company says the culprit is Yahoo. There's an error in the way Windows Phone 7 synchronises email, meaning that the phone ends up downloading 25 times as much data as it needs to.
"We have determined that an inefficiency exists in the synchronization of email between the Windows Phone Mail client and Yahoo Mail," says the company.
"This inefficiency can result in larger than expected data usage for a very small percentage of users. Microsoft and Yahoo! have worked together to identify a fix, which will be rolled out in the coming weeks."
In the meantime, says the company, there's a fix. From the Start screen of Windows Phone 7, users can click on the arrow at the top right, choose 'Settings', 'email & accounts' and then 'Yahoo! Mail'. They should then click on the setting under 'Download new content' and select a less frequent setting - if they're using the default setting of every two hours, this should be changed to 'manually'.
Clicking on the setting under 'Download email from' and selecting a shorter time range such as 'the last 7 days' should then fix the problem.