MailMessage msg = new MailMessage();
   msg.From = new MailAddress("zaccrites@att.net");
   msg.To.Add("zaccrites@att.net");
   msg.Subject = "Subject";
   msg.Body = "THIS IS A MESSAGE!";

   SmtpClient smtp = new SmtpClient("smtp.att.yahoo.com", 645);
   //smtp.DeliveryMethod = SmtpDeliveryMethod.Network;
   //smtp.UseDefaultCredentials = false;
   smtp.Credentials = new NetworkCredential("zaccrites@att.net", "password");
   smtp.EnableSsl = true;

   Console.WriteLine("Send Start...");
   smtp.Send(msg);
   Console.WriteLine("Message Sent");

<system.net>
    <mailSettings>
        <smtp deliveryMethod="Network" from="testuser@domail.com">
            <network defaultCredentials="true" host="smtp.att.yahoo.com" port="645" userName="yourUser" password="yourPass"/>
        </smtp>
    </mailSettings>
</system.net>