While it has been confirmed that Yahoo! email was the rascal that sucked the data out of your Windows Phone, Yahoo! is now apologizing for their part in the situation.  In a written statement, Yahoo! shares,

"Tens of millions of people check their Yahoo Mail from their mobile device each day, and we know they want their mobile mail experience to be fast, rich, and real-time.  While our default settings on all mobile platforms realize this approach, we have determined that an inefficiency exists in the synchronization of e-mail between Windows Phone Mail clients and Yahoo Mail, which can result in larger than expected data usage for some users."

While Yahoo and Microsoft are working to deliver a fix for this problem, Yahoo! notes that the data issue was not affecting other phone platforms. In a follow up statement to CNET, Yahoo states,

"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."

It's nice to see that the problem has been identified and a solution in the works. And while Yahoo! has apologized for the inconvenience, it almost sounds as if they are trying to shift the blame to Microsoft or, is Yahoo! simply re-assuring other platforms that this data issue isn't possible on those phones?

Source: CNET