I had the same issue. Purchased a skyrocket s2, and added 1 Gmail account and 2 yahoo accounts using pop3. (Standard default when adding email to be pushed into phone). After about 2 months, started eating data like crazy, kept getting texts saying I hit my limit, then hit my overage, ect. A quick solution was to deactivate the data. ( go into settings, wireless and internet, scroll to bottom to mobile net work and un-check "use packet data". Would just activate packet data when I really needed to use the internet, or see if I had mail. After a month of this, I called sprint, and they couldn't help. Went online, and found out it has to do with a.glitch in the gingerbread operating system
So, I downloaded "my data tracker" or something like that, and figured out it was from the email.
So here's what I figured out. One solution is to Change from using pop3 to Inappropriate. Go into your email, click the more options/ list looking button, and go onto.account settings if I remener correctly. Switch from pop3 to imap. I didn't like this option.
What's happening is that with a Gmail account, you can set incoming data settings and outing setting. (On a seperate note, Gmail is the way to go if you want emails updated quick.) On yahoo, you can only mess.with the incoming settings. If you have it the outing settings set to "copy emails to server" or something like that, it makes a copy of every email ever sent to your PHONE, regardless if you delete them on the phone or computer. When Gmail checks for new mail, it keeps sending the accumulated deleted and undeleted messages back to the phone eating the data, ergo, you might not of had any issues when you first got the phone, but it gets steadily worse as you get emails, and think there deleted for.good.
I had to delete all 3 email addressable from phone, and put em back on to figure this out. When I reloaded my Gmail, I went into account setting for Gmail, and made it so that it did not save the emails on the server.(set it to"delete emails from server when deleting" or something like that. Then I opened my Gmail, and there was an option to "load emails", so I clicked it, and goulash, every email my phone ever got came back and I deleted em for good. Had to keep clicking "load emails" a lot to get em all.
After that, things have been kosher. Means you have to delete you gotta delete your emails a lot.
Hope this saves some people from some hair pulling