There has been a nagging data usage overage going on in Windows Phone 7 since its release, and today we find out who the culprit was! Due to the great work by Rafael Rivera of Within Windows he has found that Yahoo Mail on WP7 is using up to 25x the normal amount of data usage because of it not responding to FETCH requests properly.
Visit Within Windows for all the technical details.
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).
A big congrats to Rafael for figuring this out!