How legal are signed e-mail documents?

It is common for us to get apartment applications, lease extensions, etc. via e-mail. They are signed and dated and we always request the original be mailed to us but with the e-mail do we have a legal document in hand?
I realize that it could be taken to court and fought out but I wondered if anyone knew the legal standings of such document or had heard of a court case about e-mail signatures?