Sending mail through exchange server

protected void Button1_Click(object sender, EventArgs e)
   {
       SmtpClient smtpClient = new SmtpClient("exchangeServerHTTPS", 25);
       System.Net.Mail.MailMessage message = new System.Net.Mail.MailMessage();
       try
       {
           MailAddress fromAddress = new MailAddress("operations@test.com", "From Me");
           MailAddress toAddress = new MailAddress("faxsv@test.com", "To You");
           message.From = fromAddress;
           message.To.Add(toAddress);
           message.Subject = "Testing!";
           message.Body = "This is the body of a sample message";
           smtpClient.UseDefaultCredentials = true;
           System.Net.NetworkCredential nc = CredentialCache.DefaultNetworkCredentials;
           smtpClient.Credentials = (System.Net.ICredentialsByHost)nc.GetCredential("exchangeServerHTTPS", 25, "Basic");
           smtpClient.Send(message);

           lblmsg.Text = "Email sent.";
       }
       catch (Exception ex)
       {
           lblmsg.Text = "Coudn't send the message!\n  " + ex.Message;
       }

   }