Alright folks, it appears the culprit in the ongoing phantom data problems has been fully identified. It was spotted by Rafael Rivera who managed to sniff out that Yahoo! Mail is sending about 25x as much data as it needs to, due to not handling FETCH requests properly.
Microsoft has now confirmed that this is the issue, saying:
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.
Microsoft and Yahoo! have worked together to identify a fix, which will be rolled out in the coming weeks.
In the meantime, customers can mitigate this issue by following these steps:
On the Start screen of Windows Phone 7, click on the arrow at the top right
Choose “Settings” from the app list
Choose “email & accounts”
Choose “Yahoo! Mail”
Click on the setting under “Download new content”
Select a less frequent setting. If you are using the default setting (every 2 hours), change this setting to ‘manually’
Click on the setting under “Download email from”
Select a shorter time range. If you are using the default setting (the last 2 weeks), change this setting to ‘the last 7 days’
Microsoft has also identified an issue with our Exchange ActiveSync [EAS] email synchronization protocol that may affect a very small percentage of users
This issue can be caused by email that exceeds the mail server email size setting or is sent to an invalid email address
This issue can potentially affect email services that utilize the EAS protocol – Outlook and Gmail for example
The immediate mitigation is for users to delete mail that appears “stuck” in the outbox
This is currently scheduled to be fixed in a near-term end user update from Microsoft
“Near-term end user update?” Let’s hope that it’s very near-term indeed. People are not happy about this, and a fix needs to come out ASAP.