Some deep digging by self-proclaimed Microsoft software deconstructor Rafael Rivera claims to have stumbled upon the reason for some Windows Phone 7 devices using up large amounts of "phantom" data, unbeknownst to their owners.

The culprit Yahoo Mail.

Rivera examined some of the IMAP packets coming into the device and discovered that Yahoo's IMAP server was not responding to the phone's requests to fetch new messages with the correct response. Instead of sending commands that were just a few lines in length, the servers would send back code chunks that Rivera says are about 25 times larger than they need to be.

"This can also be read as: Your phone is downloading ~25 times as much data as it normally would, had you sided with another e-mail provider," Rivera said.

Microsoft confirmed Rivera's findings this evening, saying that there was an "inefficiency" between the Windows Phone Mail client and Yahoo's e-mail service. The company is instructing affected users to slow mail fetches until it can offer up a software fix.

Problems with Windows Phone 7 devices slurping up excessive amounts of data were first reported shortly after the launch of the platform, but not all phones experienced the issue. Responding to reports of the problem earlier this month, Microsoft pledged to launch an investigation into the issue, which it pinned on an unnamed third party a week later. In any case, Microsoft downplayed the scale of the issue, saying that "a small (low single-digit) percentage" of Windows Phone 7 users had reported the issue to the company.

(via Windows Phone Secrets)

Update at 8:46 p.m. PT: Microsoft has confirmed to CNET that the problem exists, and has provided the following instructions to Windows Phone 7 users until a fix arrives.

Long story short, Microsoft is encouraging users to set the phone to check for e-mail from their Yahoo accounts less often. Microsoft also says it's working on a fix to another e-mail related bug that has some messages sitting in the outbox instead of being sent:

This inefficiency can result in larger than expected data usage for a very small percentage of users.

&bull' Microsoft and Yahoo! have worked together to identify a fix, which will be rolled out in the coming weeks. &bull' In the meantime, customers can mitigate this issue by following these steps:

1. On the Start screen of Windows Phone 7, click on the arrow at the top right 2. Choose "Settings" from the app list 3. 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'

Microsoft has also identified an issue with our Exchange ActiveSync [EAS] email synchronization protocol that may affect a very small percentage of users.

&bull' This issue can be caused by email that exceeds the mail server email size setting or is sent to an invalid email address&bull' This issue can potentially affect email services that utilize the EAS protocol - Outlook and Gmail for example &bull' The immediate mitigation is for users to delete mail that appears "stuck" in the outbox &bull' This is currently scheduled to be fixed in a near-term end user update from Microsoft