Checking mail on mobile data
If you go into Settings -> Data Usage you can see which apps are using the most data. It's possible to turn them off using data in the background by clicking on the app and then selecting "Restrict Background Data". See this and this for pictures and further detail.

Also if it's just email you're using you can probably turn it off automatically checking depending on the client and email service you're using. If you're using Gmail then go into the Gmail app, go to the settings for your account and turn off "Sync Gmail".