protected void btnsubmit_Click(object sender, EventArgs e
{
System.Net.Mail.MailMessage msg = new
System.Net.Mail.MailMessage(txtfrom.Text, txtname.Text,
txtSubject.Text, txtmessage.Text);

        System.Net.Mail.SmtpClient mysmtp = new
SmtpClient("192.168.1.80");
        try
        {
            mysmtp.Send(msg);
            lblmsg.Text = "mail sent";
        }
        catch (Exception ex)
        {
            lblmsg.Text = ex.Message;
        }
    }
}

OR

MailMessage message = new MailMessage ();
        message.From = ;
        message.To = ;
        message.Subject = "Scheduled Power Outage";
        message.Body = "Our servers will be down tonight.";
        SmtpMail.SmtpServer = "localhost";//name of the server
        SmtpMail.Send (message);


MailMessage and SmtpMail are classes defined in the .NET 
Framework Class Library's  System.Web.Mail namespace.