Microsoft's own Mail app and Yahoo! Mail have been implicated by the software giant as the cause of the Windows Phone 7 bug that has caused some handsets to rack up users' data transfer tallies.

Microsoft last night blamed an "inefficiency" in the way its Windows Phone Mail app talks to Yahoo! Mail. The result: "larger than expected data usage for a very small percentage of users".

Essentially, the app appears to be downloading rather more email than it needs to, pulling in two weeks' worth of messages at a time, even if they have been read.

The problem seems to affect IMAP accounts rather than POP, another common email transfer protocol. According to a report by website Windows Phone Secrets, Yahoo!'s Windows Mobile-oriented IMAP servers incorrectly implement IMAP's Fetch command, though Yahoo! has yet to confirm this.

Microsoft said it and Yahoo! were working on a fix which will be released "in the coming weeks".

In the meantime, reduce the frequency at which Mail checks Yahoo! Mail for new messages and the period over which its bulk download feature pulls in messages. ®

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