ASP.NET

public void SendMail()
     {

         try
         {
             String strTo = "vuyiswam@tshwane.gov.za"; ;
             String strFrom = "vuyiswamb@gmail.com";
             String strSubject = "Hi vuyiswa";
             String strbody = "I did it yahooo";
             SmtpMail.SmtpServer = "smtp.gmail.com";

             MailMessage mail = new MailMessage();

             mail.To = strTo;

             mail.From = strFrom;

             mail.Subject = strSubject;

             mail.Body = strbody;

            SmtpMail.Send(mail);
         }
         catch (Exception e)
         {
             MessageBox.Show(e.Message);

         }

     }

Object reference not set to an instance of an object. 
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error: 


Line 93:             lblTotal.Text = [String].Format("{0:C2}", +_ordersBLL.Total.ToString())
Line 94:           
Line 95:             cbsPayment.Checked = _ordersBLL.sPayment
Line 96:             cbsFood.Checked = _ordersBLL.sFood
Line 97: 

string filename = f.Name;<br />
            Response.ContentType = "application/octet-stream";<br />
            Response.AddHeader("content-disposition", "attachment; filename = " + str);<br />
            Response.BinaryWrite(b);<br />
            Response.End();