Right after the launch of Windows Phone 7, users started to report about abnormal high idle data usage they haven't experienced with any handsets before and Microsoft followed-up earlier this year and confirmed that it started to investigate the phantom-data cause. According to BBC and fellow MVP reports, some users have complained that their phone has send "between 30 and 50 MB of data every day; an amount that would devour a 1 GB allowance in 20 days". Other users have noticed that the phone's "idle data usage is around 2 - 5 MB per hour" and interesting enough, most reports and complaints seemed to came from the U.S.
While I haven't noticed any abnormal data usage on any of my Windows Phone 7 devices (I already easily hit the 5 GB mark with my regular use), Microsoft said later that the root cause was found but only said that "its investigation found that most problems were caused by an unnamed 'third party' service". However, Microsoft denied to give any further details and therefore it wasn't clear if it was an inbuilt third party service, like Microsoft's Facebook integration or a downloaded third party application like Facebook's very own Windows Phone 7 app.
Nevertheless, as it turned out today, it was neither an internal social media service nor a later downloaded third party application but Microsoft's very own integration of Yahoo! Mail! To make it easy for users to setup Windows Phone 7, Microsoft has pre-included a wide range of popular E-Mail services where users have to enter the E-Mail address and password only and all the rest is automatically configured by the Windows Phone 7; for instance Windows Phone 7 comes out of the box with support for Windows Live, Outlook, Yahoo! Mail and Gmail as well as ODMs and carriers can add even further E-Mail services to the list of predefined services.
According to information Paul Thurrott received from Microsoft, the Windows Phone 7 maker admitted yesterday that "it has determined that an inefficiency exists in the synchronization of E-Mail between the Windows Phone Mail client and Yahoo! Mail" can continued that "this inefficiency can result in larger than expected data usage for a very small percentage of users". Furthermore Microsoft confirmed that "Microsoft and Yahoo! have worked together to identify a fix, which will be rolled out in the coming weeks" (most likely together with the expected NoDo update which might also explains why NoDo wasn't released in January as expected/rumored first). In the meantime, Microsoft recommends to tweak the Yahoo! Mail settings to manually download E-Mails instead of using the default settings which polls E-Mails every 2 hours. Also Microsoft recommends to use a shorter time range of "the last 7 day" instead of the default setting of the last 2 weeks.
The E-Mail settings can be changed by choosing "Settings" from the app list, tapping "email & accounts", choosing "Yahoo! Mail" and tapping the setting under "Download new content".
Furthermore, Microsoft has also identified an issue with its Exchange ActiveSync [EAS] E-Mail synchronization protocol that may affect another very small percentage of users. This issue can be caused by E-Mail that exceeds the mail server E-Mail size setting or is sent to an invalid E-Mail address. This issue can potentially affect E-Mail services that utilize the EAS protocol - Outlook and Gmail for example and the immediate mitigation is for users to delete mail that appears "stuck" in the outbox.
Like the Yahoo! Mail bug, this issue is currently scheduled to be fixed in a near-term end user update from Microsoft!
UPDATE: I've just received an official information from Microsoft's PR where the Yahoo! Mail and Exchange ActiveSync bugs were officially confirmed! And indeed, Microsoft suggests to change Yahoo! sync settings - as mentioned above - as an interim-workaround.
Cheers ~ Arne