I'd suggest getting a data plan with enough data so that you don't have to worry about your email putting you close to your limit. Failing that, go into your mail settings and set it to check for new messages manually - then you can check your email on your schedule, probably when you have a wifi connection, instead of the phone's schedule. Assuming that your unlocked phone is also jailbroken, you could also use a utility to just turn Edge off, which would stop Mail from being able to connect over the Vodafone network.