Microsoft on Monday said that excess data charges on Windows Phone 7 devices were caused by a synchronization issue between the Windows Phone Mail client and Yahoo Mail.
The announcement comes about a week after Microsoft said a third-party app was causing the overage charges. Microsoft said last month that it was investigating complaints from Windows Phone 7 users who said their devices were arbitrarily sending out data for no reason.
"We have determined that an inefficiency exists in the synchronization of e-mail between the Windows Phone Mail client and Yahoo Mail," Microsoft said in a statement. "This inefficiency can result in larger than expected data usage for a very small percentage of users."
Microsoft said it worked with Yahoo to identify a fix, which will roll out in the coming weeks. In the meantime, the company offered up a manual fix: Click the arrow on the top right of the Windows Phone 7 start screen; Choose "settings"; Choose "e-mail and accounts"; Choose "Yahoo Mail"; click on the setting under "download new content"; select a less frequent setting - those on the default, two-hour setting should choose "manually"; click on the setting under "download e-mail from"; and select a shorter time range - if you're on the default, choose "the last 7 days."
A Yahoo spokesperson said the issue was "specific to how Microsoft chose to implement IMAP for Yahoo Mail and does not impact Yahoo Mail on these other mobile devices."
"Yahoo has offered to provide Microsoft a near-term solution for the implementation they chose, and is encouraging Microsoft to change to a standard way of integrating with Yahoo Mail, which would result in a permanent fix," Yahoo concluded.
Microsoft said it also identified an issue with its Exchange ActiveSync e-mail synchronization that might affect a small percentage of users. It can be caused by very large e-mails or those that are sent to an invalid e-mail address, and can affect e-mail services on the EAS protocol, like Outlook and Gmail. The immediate fix is to delete the e-mails that are stuck in outbox limbo; Microsoft said it will be permanently fixed soon via an end user update.
Editor's Note: This story was updated at 10:15pm Eastern with comment from Yahoo.