Windows Phone 7 users can blame Yahoo Mail for their mysteriously high 3G data usage, the company revealed to Windows Phone Secrets yesterday.

Microsoft didn’t admit Yahoo was the problem until Within Windows’ Rafael Rivera did some sleuthing and confirmed that Yahoo’s mail server was sending around 25 times as much data as it needed to. Previously, Microsoft would only say that an unnamed third-party was the root of the issue.

Many thought that Microsoft was only covering for its own software issues by blaming a third-party, but now it’s clear that Yahoo is indeed guilty. Still, Microsoft isn’t completely blameless, as it could have provided users some relief earlier by admitting Yahoo was the problem and offering some tips on how to minimize the issue. For now, Microsoft recommends changing your Yahoo Mail account to manually update, and reducing the time range of emails synchronized.

Microsoft says that it has worked together with Yahoo to fix the issue, and a software update will be coming in the next few weeks. Additionally, Microsoft says that it has found a problem with its Exchange ActiveSync protocol that could affect Outlook and Gmail users. A fix for that is one the way as well.

Update: Yahoo has responded to the news with the following statement, which pushes the blame back to Microsoft:

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.

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