Initially, Microsoft’s investigation had stated that a third party was to blame, but the company didn’t name that third party.
If you missed the whole phantom data problem, it involved Windows Phone 7 sending mysterious chunks of data without the mobile owner’s knowledge, running up their data allowance.
Paul Thurrott at Windows Phone Secrets posted up the confirmation from Microsoft that Yahoo Mail was indeed to blame.
Microsoft stated: “We have determined that an inefficiency exists in the synchronization of email between the Windows Phone Mail client and Yahoo Mail. This inefficiency can result in larger than expected data usage for a very small percentage of users.”
Both companies are now working together to concoct a fix, which will apparently be due in the next few weeks.
Until then, MS notes that users can mitigate the problem by going to Email & Accounts, under Settings from the app list, selecting Yahoo Mail, and clicking on the setting under Download New Content. You can then change this to a less frequent amount (the default is every two hours, which you can change to “manually”).
You can also alter the setting under Download Email From to a shorter time range, i.e. the “last 7 days” instead of the “last 2 weeks”.