Yesterday, Microsoft issued a press info where the Windows Phone 7 maker said that it identified the root cause of the so called "phantom data" problem and blamed (Windows Phone 7's) Yahoo! Mail (integration) for it. And within less than 24 hours, Yahoo! responded with a statement Pocket-lint has received. According to Yahoo!, "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."
Nevertheless, "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."
I think it's safe to say that nobody expected that Yahoo! contributed the (faulty) code but it was quite clear that it was just Microsoft's very own Yahoo! Mail Windows Phone 7 integration. However, it's indeed remarkable that Microsoft named Yahoo!'s mail service as the faulty cause, without mentioning that Microsoft itself might have coded something wrong which unfortunately concerns Yahoo! Mail.
In any case, hopefully both will figure out a workaround soon because the end-user has to pay the bill for possible overcharges! This should be fixed latest with the upcoming NoDo update for Windows Phone 7, everything else isn't acceptable.
Cheers ~ Arne