The secret culprit behind the Windows Phone 7 phantom data issue is ... *drumroll please* ... Yahoo! Mail.
Windows Phone 7 guru Rafael Rivera confirms what insiders have been telling me for more than a week:
Scary. Yahoo is sending ~25 times as much data as it needs to. (This can also be read as: Your phone is downloading ~25 times as much data as it normally would, had you sided with another email provider.)
To workaround this, I strongly recommend Yahoo mail users reconfigure the phone to not transmit data via a cellular connection (Settings –> Cellular –> Data roaming options). As an alternative, you can set your Yahoo account to only Download new content only on manual trigger (Yahoo Mail –> Settings –> Sync Settings).
The problem is down to Yahoo’s IMAP server (winmo.imap.mail.yahoo.com) and the fact that it doesn't respond to FETCH requests correctly.
No word from Yahoo! on this yet.
Nice bit of sleuthing there Rafael ... well done!