How 2 send mail using asp.net?

ASP.NET Email Tutorial
This tutorial will show you how to send a simple email message using ASP.NET 2.0 and C#
Sending a email using ASP.NET 2.0 and C# 2.0 is actually very simple. First, you will need to import the System.Net.Mail namespace.The System.Net.Mail namespace contains the SmtpClient and MailMessage Classes that we need in order to send the email.
using System.Net.Mail; // Importent
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protected void btnSubmit_Click(object sender, EventArgs e){
try{
MailMessage message = new MailMessage(txtFrom.Text, txtTo.Text, txtSubject.Text, txtBody.Text);SmtpClient emailClient = new SmtpClient(txtSMTPServer.Text);emailClient.Send(message);litStatus.Text = "Message Sent";}catch (Exception ex){
litStatus.Text=ex.ToString();}}
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The front end .aspx page looks something like this:

To
From
SMTP Server
Subject
Body
Action
Status

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The flow for the code behind page is as follows.
using System;using System.Data;using System.Configuration;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.Net.Mail;public partial class _Default : System.Web.UI.Page{
protected void Page_Load(object sender, EventArgs e){}protected void btnSubmit_Click(object sender, EventArgs e){
try{
MailMessage message = new MailMessage(txtFrom.Text, txtTo.Text, txtSubject.Text, txtBody.Text);SmtpClient emailClient = new SmtpClient(txtSMTPServer.Text);emailClient.Send(message);litStatus.Text = "Message Sent";}catch (Exception ex){
litStatus.Text=ex.ToString();}}}

It is not possible to change SMTPServer (in codebehind ) hence change in web.config :

in code behind :
add String str = ConfigurationManager.AppSetting("SMTPServer");