How To Send An Email Using Visual Studio 2010 And In That I Want to Change the From Address?

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C#

ASP.NET

Hi i want to know the code or the way , that i have designed a send an email using c#, in that i want to change the from address which is visible to the recipient. for example :
if i send a message to sss@gmail.com from sar@gmail.com means.
the sss@gmail.com receive the msg from dss@gmail.com instead of sar@gmail.com.
how to make this from address changes .my code is:

try
            {
                MailMessage Msg = new MailMessage();
                // Sender e-mail address.
                Msg.From = new MailAddress(txtUsername.Text);
                // Recipient e-mail address.
                Msg.To.Add(txtTo.Text);
                Msg.Subject = txtSubject.Text;
                Msg.Body = txtBody.Text;
                // your remote SMTP server IP.
                SmtpClient smtp = new SmtpClient();
                smtp.Host = "smtp.gmail.com";
                smtp.Port = 587;
                smtp.Credentials=new System.Net.NetworkCredential(txtUsername.Text,txtpwd.Text);
                smtp.EnableSsl = true;
                smtp.Send(Msg);
                Msg = null;
<pre lang="xml">Page.RegisterStartupScript("UserMsg", "<script>alert('Mail sent thank you...');if(alert){ window.location='default.aspx';}</script>");
            }
            catch (Exception ex)
            {
            Console.WriteLine("{0} Exception caught.", ex);

}
}