Microsoft has, after much digging, found the culprit for a data leak within its Windows Phone 7 operating system that was causing some users to have much higher than expected data costs.

What app or service was to blame? The finger seems to have fallen on Yahoo! Mail and an inefficiency that “exists in the synchronization of email between the Windows Phone Mail client and Yahoo! Mail.”

According to Microsoft, “This inefficiency can result in larger than expected data usage for a very small percentage of users,” reports WindowsPhoneSecrets.com.

Those worried shouldn't be as Microsoft and Yahoo! are supposedly working together to identify a fix, which will be rolled out in the coming weeks.

In the meantime the site details ways WP7 users can mitigate this issue by following these steps:

On the Start screen of Windows Phone 7, click on the arrow at the top right

Choose “Settings” from the app list

Choose “email & accounts”

Choose “Yahoo! Mail”

Click on the setting under “Download new content”

Select a less frequent setting. If you are using the default setting (every 2 hours) change this setting to "manually"

Click on the setting under “Download email from”

Select a shorter time range. If you are using the default setting (the last 2 weeks), change this setting to "the last 7 days"

This info comes hot on the heels of the news that Microsoft is rumoured to be releasing a new update for the mobile operating system on 7 February, just days before Mobile World Congress kicks off later in the month.

We will keep you posted.

UPDATE: Yahoo has got in touch with Pocket-lint offering this statement:

“Yahoo! Mail is widely available on tens of millions of mobile phones, including those running on Apple iOS, Android, Nokia Symbian, and RIM. The issue on the Windows Phones is 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." 

- 72 Windows Phone 7 tips and tricks