erro no envio do correio para a excepção smtp?

StringBuilder emailMessage = new StringBuilder();
emailMessage.Append("Dear Payment Team ,");
emailMessage.Append("<br><br>Please find the Payment instruction");    

try
{
    MailMessage Msg = new MailMessage();
    // Sender e-mail address.
    Msg.From = new MailAddress("[email protected]");
    // Recipient e-mail address.
    Msg.To.Add("[email protected]");
    Msg.CC.Add("[email protected]");
    Msg.Subject = "Timesheet Payment Instruction updated";
    Msg.IsBodyHtml = true;
    Msg.Body = emailMessage.ToString();
    SmtpClient smtp = new SmtpClient();
    //smtp.EnableSsl = true;

    smtp.Host = ConfigurationManager.AppSettings["HostName"];
    smtp.Port = int.Parse(ConfigurationManager.AppSettings["PortNumber"]);
    smtp.Send(Msg);
    Msg = null;
    Page.RegisterStartupScript("UserMsg", "<script>alert('Mail has been sent successfully.')</script>");
}
catch (Exception ex)
{
    Console.WriteLine("{0} Exception caught.", ex);
}

adicionei este código na web.config

<appSettings>
    <add key="HostName"   value="The host name as given by my company" />
    <add key="PortNumber" value="25" />
</appSettings>

continuo a ter uma excepção, tentei mudar o número do porto como especificado, mas sem sucesso

Pormenor Das Excepções

  System.Net.Mail.SmtpException was caught
  Message=Failure sending mail.
  Source=System
  StackTrace:
       at System.Net.Mail.SmtpClient.Send(MailMessage message)
       at PAYMENT_DTALinesfollowup.Sendbtn_Click(Object sender, EventArgs e) in d:\AFSS-TFS\AFSS\Code\ERPNET\PAYMENT\DTALinesfollowup.aspx.cs:line 488
  InnerException: System.Net.WebException
       Message=Unable to connect to the remote server
       Source=System
       StackTrace:
            at System.Net.ServicePoint.GetConnection(PooledStream PooledStream, Object owner, Boolean async, IPAddress& address, Socket& abortSocket, Socket& abortSocket6, Int32 timeout)
            at System.Net.PooledStream.Activate(Object owningObject, Boolean async, Int32 timeout, GeneralAsyncDelegate asyncCallback)
            at System.Net.PooledStream.Activate(Object owningObject, GeneralAsyncDelegate asyncCallback)
            at System.Net.ConnectionPool.GetConnection(Object owningObject, GeneralAsyncDelegate asyncCallback, Int32 creationTimeout)
            at System.Net.Mail.SmtpConnection.GetConnection(ServicePoint servicePoint)
            at System.Net.Mail.SmtpTransport.GetConnection(ServicePoint servicePoint)
            at System.Net.Mail.SmtpClient.GetConnection()
            at System.Net.Mail.SmtpClient.Send(MailMessage message)
       InnerException: System.Net.Sockets.SocketException
            Message=A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond 125.63.68.148:25
            Source=System
            ErrorCode=10060
            NativeErrorCode=10060
            StackTrace:
                 at System.Net.Sockets.Socket.DoConnect(EndPoint endPointSnapshot, SocketAddress socketAddress)
                 at System.Net.ServicePoint.ConnectSocketInternal(Boolean connectFailure, Socket s4, Socket s6, Socket& socket, IPAddress& address, ConnectSocketState state, IAsyncResult asyncResult, Int32 timeout, Exception& exception)
            InnerException: