ทำส่งเมล์ไม่ได้ครับ ทำตามแล้ว ก็ยัง System.Runtime.InteropServices.COMException: The "SendUsing" configuration value is invalid.

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Web.Mail;
using System.Net;

public partial class Default6 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        MailMessage myMail = new MailMessage();

        myMail.To = this.TextBox1.Text;
        myMail.From = "<" + this.TextBox4.Text + ">" + this.TextBox4.Text;
        myMail.Subject = this.TextBox2.Text;

        myMail.BodyFormat = MailFormat.Html;
        myMail.Body = this.TextBox3.Text.Replace("\n", "<br>");

        SmtpMail.Send(myMail);
        myMail = null;
    }
}