Problem in retrieving email of registered user

Question

i am using create user wizard to create new users. i don't want to allow enter the which has been already registred, i use this code for restricting the user to enter the duplicate email but it's not working .

private bool UserExists(string usernameEmail)
    {
        if (Membership.FindUsersByEmail(usernameEmail) != null)
        {
            return true;
        }

        return false;
    }

    protected void RegisterUser_NextButtonClick(object sender, WizardNavigationEventArgs e)
    {
        if (e.CurrentStepIndex == 0)
        {
            if (RegisterUser.Email.Trim() == "" || UserExists(RegisterUser.Email))
            {
                lblmsg.Text = "This Email Is already Exist ! Please try Another Email";
                lblmsg.ForeColor = Color.Green;
                e.Cancel = true;
            }
            else
            {
                e.Cancel = false;
            }
        }
    }

here lblmsg is label for displaying the error message and RegisterUser is ID of CreateUserWizard.