The reason: Yahoo! seems to be sending and receiving far more data than it really needs to. In fact, on Rafael’s count, it sends roughly 25 times more data than it needs to. Yikes!
Rafael provides a fair amount of detail at his site, which covers the information sent to/from Windows Phone. The discussion covers specific IMAP4 issues and implementations, including STARTTLS extension.
Although, to me, the best fix for this would be to stop using Yahoo! as an e-mail provider and switch to a more efficient service like Hotmail or Gmail, Rafael does offer a different approach as well. That is, to simply not transmit data via a cellular connection. Although, this seems a bit rash to me, not on his part, but on Yahoo!’s. The only way, apparently, to use their service efficiently is to use it via a wireless (i.e., non-AT&T or T-Mobile) connection? That’s fairly lame on Yahoo’s part. It also means your mobile device just became less useful, because of Yahoo!. Terrific.
Yahoo! may indeed be one of the culprits in the recent Windows Phone data leaks. However, it may not be the only source of a leak. As I currently don’t have access to a Windows Phone device to tear into to start dissecting data transfers, I can’t make any firm decisions. However, if Microsoft wishes to send me a developer device, I would more than gladly dissect all that I can. But, I won’t hold my breath.
Hopefully, Yahoo! will work out their data usage issues in the near future, and we won’t have to worry about this anymore. Until then, just don’t read your Yahoo! e-mail on the fly and you’ll be fine.