Microsoft this evening belatedly admitted that Yahoo! Mail was in fact the data leak culprit in Windows Phone, just hours after Rafael had already proven that to be the problem. Here’s the note from Microsoft, which also includes information about another, semi-unrelated issue for some reason.
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.
Microsoft has also identified an issue with our Exchange ActiveSync [EAS] email synchronization protocol that may affect a very small percentage of users
Full Story at Windows Phone Secrets