Textbox1  ( assign the name in the textbox properties like.... txtto )
Textbox2 ( assign the name in the textbox properties like..... txtsub )
Textbox3 ( assign the name in the textbox properties like..... txtmsgbody
           and change the textmode as  multilevel )
Button1  ( assign the name in the button properties like.... btnsend )
Label1  (  assign the name in the label properties like.... lblmsg)

protected void btnsend_Click(object sender, EventArgs e)
    {
       MailMessage mm = new MailMessage();
        try
        {
            mm.From = "gopiyadavalli@yahoo.com"; // your mail ID
            mm.To =txtto .Text.ToString();
           

            mm.Body = txtmsgbody.Text.ToString();
            mm.Subject = txtsub.Text.ToString();
            System.Web.Mail.SmtpMail.Send(mm); 
            lblmsg.Text = "Email successfully sent.";
        
        }
        catch (Exception ex)
        {
            lblmsg.Text = "Send Email Failed." + ex.Message;
        }
   
    }