We have determined that an inefficiency exists in the synchronization of email between the Windows Phone Mail client and Yahoo! Mail.This inefficiency can result in larger than expected data usage for a very small percentage of users.
1. On the Start screen of Windows Phone 7, click on the arrow at the top right2. Choose “Settings” from the app list3. Choose “email & accounts”4. Choose “Yahoo! Mail”5. Click on the setting under “Download new content”6. Select a less frequent setting. If you are using the default setting (every 2 hours), change this setting to ‘manually’7. Click on the setting under “Download email from”8. Select a shorter time range. If you are using the default setting (the last 2 weeks), change this setting to ‘the last 7 days’9. Microsoft has also identified an issue with our Exchange ActiveSync [EAS] email synchronization protocol that may affect a very small percentage of users
This is currently scheduled to be fixed in a near-term end user update from Microsoft