求教UDP打洞高手解决思路

求教UDP打洞高手
公网Server S能接受到某一公司内网主机E1发来的登陆消息，但是S返回的消息，E1无法接收。是否是公司网关拦截导致的？
代码如下。S端线输入SINIT,然后E1端运行E1Init，然后E1无法接受返回消息。

      static void Main(string[] args)
        {
         
                string serveraddr = "116.233.234.159";
                int port = 8600;
                long e1Addr = 0, e2Addr = 0;
                int e1Port = 0, e2Port = 0;
                System.Net.Sockets.UdpClient u = new System.Net.Sockets.UdpClient(port);
                while (true)
                {
                    string cm = Console.ReadLine();
                    if (cm == "SINIT")
                    {
                        while (true)
                        {
                            System.Net.IPEndPoint ep = new System.Net.IPEndPoint(0, 0);
                            byte[] msg = u.Receive(ref ep);
                            if (Encoding.ASCII.GetString(msg) == "Hello1")
                            {
                                e1Addr = ep.Address.Address;
                                e1Port = ep.Port;
                                for(int i=0;i<1000;i++)
                                u.Send(Encoding.ASCII.GetBytes("OK"), 2, ep);
                            }