I've already read on various forums that Android phones are well known for eating up data. I have a new HTC Sensation XL. Whereas I used about 13MB per day with my old iPhone, I am averaging between 25MB and 40MB per day with my HTC phone. All the comments I've read put the blame on the Android e-mail program and two data monitoring apps I've downloaded have concluded the same. Does anyone have the same experienced? I've asked HTC to comment in the meantime. Why would their e-mail program use so much more data than an iPhone. I have just downloaded the Yahoo and Hotmail apps and might well test them out, with the native e-mail program turned off, to see if that restores my data usage to its usual levels.
I would recommend you download Onavo; it's a data tracking app that tells you how much data all your individual apps are using. It might not be the email app causing the problem, the Background Data and Auto-Sync features in 'Accounts and Sync' are often the worst culprits. Onavo also lets you restrict certain apps to wifi only, which I would definitely recommend for big downloaders like the marketplace.
I already have Onavo and that's the app that keeps telling me to kill my e-mail my program. HTC have suggested extending the frequency that the e-mail program checks for messages. I will try that, though I'm suspicious that it's the way the e-mail program operates that's getting through so much data.