Windows Phone Secrets reports that Microsoft has named Yahoo as the party responsible for a WP7 app which was generating high volumes of data traffic, stating that “an inefficiency exists in the synchronisation of email between the Windows phone Mail client and Yahoo Mail.” It was previously reported that WP7 devices were sending large amounts of data when in an idle state, significantly eating into the data allowances of customers without unmetered tariffs. This would now appear to have been devices synchronising with the Yahoo Mail server. Apparently, a fix will be rolled out to users “in the coming weeks,” although in the meantime customers can reduce the amount of data transferred by manually adjusting the app settings.

According to The Register, Yahoo subsequently said that the issue is related to the specific IMAP implementation used in the Windows Phone Mail app, and that the glitch “does not impact Yahoo Mail on other devices.”

While the issue is believed to have affected a “small percentage of users,” it is still somewhat embarrassing for Microsoft and Yahoo. The app was one of the early titles available for the platform from a time when Microsoft was working with a limited set of partners to bring products to market, and the app also passed through the Microsoft’s approvals process, which should have ensured it did not include any significant glitches.

Microsoft also noted that there was “an issue with our Exchange ActiveSync email synchronisation protocol,” caused by emails which exceed the mail server email size or which are sent to an invalid address. The issue, which can affect email services based on EAS including Outlook and Gmail, can be resolved if users “delete mail that appears “stuck” in the outbox.” This issue is set to be resolved by a “near-term end-user update from Microsoft.”