The e-mail can be viewed as an offer to treat and is not legally binding by any means. Same when you go to a supermarket and see a price for a good. It can change both ways... What is legally binding is a written contract with signatures that confirms the understanding of the document (in your case a working contract).

This said (and for most of us who have a few working years of experience behind us will know this) never resign a job before you have signed a new employment contract if you don't want to be left without a job needlessly.