I don't know if this applies in your situation, but a friend who had set up an outlook account with a large directory tree found a similar problem - the mail program parses the whole mail system to find any changes to your mail. Consumes HUGE amounts of data.
A few suggestions:
Make sure you are not pushing email updates to your phone - set it to fetch email only when you want.
Change email program to another.
Check to make sure no other programs are downloading data over your cellular network - turn them off in settings, if necessary so they only download on wi-fi.
I'm sure others will stop by with further suggestions.