My parents and I share our data plan, and we had only gone over our limit once in the last year. All of the sudden, we started receiving alerts that we had used 50%, then 75%, then 90% in rapid succession. We looked at the usage on all of our phones, and I found out that my Thunderbolt had used 3GB in less than ten days. Nearly all of that was attributed to the native Mail app that I only use for my POP3 account. I started taking some screenshots, and when I went into the gallery, I stumbled onto the fact that there were thousands and thousands of copies of the same image in my downloaded images folder that was attached to one of the emails in my inbox. The Mail app had apparently downloaded and re-downloaded the attached image on that email many thousands of times... WHYYYYYYYY does it use the gallery in the first place, and WHYYYYYYYY can't it remember that it has already downloaded the attachment? This is *incredibly* stupid and just cost us an overage charge. I had not been paying too much attention to my usage because as I said, we almost never went over the limit. Then, all of the sudden, it skyrocketed. It's one thing to say that I should have kept a better eye on it, but this is the kind of thing that makes ya facepalm.
Apparently, every single time that the native Mail app on the Thunderbolt syncs with a POP3 account (sounds like this happens for all phones with this native Mail app), it downloads a NEW copy of the attachment, hence the thousands of duplicated images. If your Mail app is responsible for your data usage jumping to insane levels, I'd recommend taking a look at your downloaded images folder. It's probably full of duplicated images.
Over the last hour or so, I've been digging further into the forums, and there are more settings available in the app for Exchange accounts that let you set whether it downloads the attachments when it syncs, but as far as I could find, there are no workarounds using the native app for "run-o-the-mill" accounts other than to probably either set the phone to delete the emails from the server after downloading them (which I do NOT want to do, and unfortunately, IMAP is not an option for this account) or to set it to manually sync when I open the app and just deal with it duplicating each time I open it.
Other options would be to route that account through gmail, which is what I intend to do, or to install a 3rd party app and remove the account from the native app.