'Validates that a user exists with the given e-mail address.'

def clean_email(self):

    email = self.cleaned_data['email']
    self.users_cache = User.objects.filter(email__iexact=email)
    if (len(self.users_cache) == 0):
        raise forms.ValidationError(_("That    e-mail    address    doesn't    have    an    associated    user    account.    Are    you    sure    you've    registered?"))
    return email
