Microsoft has said so-called "phantom data" leakages from Windows Phone 7 devices was the result of an issue with Yahoo Mail.
Windows Phone 7 users started complaining at the start of the year they had been charged for data that simply did not match up to how much they had actually used.
Some said despite not having used much of their 3G allowance, they had been warned by their providers about going over or close to their limit.
"We have determined that an inefficiency exists in the synchronisation of email between the Windows Phone mail client and Yahoo Mail," Microsoft said.
"This inefficiency can result in larger than expected data usage for a very small percentage of users."
Initially, Microsoft said an unnamed third-party was responsible for the data leakage.
It appears Microsoft has acted after blogger Rafael Rivera discovered there was an issue with the Yahoo Mail service, meaning it sent more data than necessary.
Rivera said there was an error in the code telling Windows Phone 7 how to retrieve messages from the Yahoo service.
"Yahoo's IMAP [Internet Message Access Protocol] server (winmo.imap.mail.yahoo.com) does not respond to FETCH requests correctly," Rivera wrote in a blog post.
"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."
Yahoo could not be reached for comment at the time of publication.
