I am developing a new App that uses Push Notifications. I completed the development of the App itself and I can send Push Notifications from the CloudKit and receive it on my phone. My problem is sending the Push Notifications from our service API to Apple. If I use the production address gateway.push.apple.com, I can send messages, but they don't work for my new App because it is in development and not yet released. If I use the sandbox address api.sandbox.push.apple.com, I get the following exception: System.Net.Sockets.SocketException (10061): No connection could be made because the target machine actively refused it. [::ffff:17.188.143.98]:2195 at System.Net.Sockets.Socket.DoConnect(EndPoint endPointSnapshot, SocketAddress socketAddress) at System.Net.Sockets.Socket.Connect(EndPoint remoteEP) at System.Net.Sockets.Socket.Connect(IPAddress[] addresses, Int32 port) --- End of stack trace from previous location --- at System.Net.Sockets.Socket.Connect(IPAddress[] addresses, Int32 port) at System.Net.Sockets.Socket.Connect(String host, Int32 port) at System.Net.Sockets.TcpClient.Connect(String hostname, Int32 port) at System.Net.Sockets.TcpClient..ctor(String hostname, Int32 port) The service API for sending Push Notifications was developed in C# .NET 8.0. Constants: private readonly string _hostname = "api.sandbox.push.apple.com"; private readonly int _port = 2195; The code for sending a message. public async Task Send(string? subtitle, string? title, string message, string deviceId) { var payload = "{ \"aps\": { " + "\"alert\": { " + "\"title\": \"" + title + "\", " + "\"subtitle\": \"" + subtitle + "\", " + "\"body\": \"" + message + "\"" + "} " + "} " + "}"; try { Debug.WriteLine($"apple api request: {payload}"); using var client = new TcpClient(_hostname, _port); var sslStream = new SslStream(client.GetStream(), false, new RemoteCertificateValidationCallback(ValidateServerCertificate)); var certificatesCollection = new X509Certificate2Collection(_certificate); await sslStream.AuthenticateAsClientAsync(_hostname, certificatesCollection, SslProtocols.Tls12 | SslProtocols.Tls13, true); var array = DataToBytes(deviceId, payload); sslStream.Write(array); sslStream.Flush(); client.Close(); } catch(Exception exception) { Debug.WriteLine(exception); } } The certificate that is used in this code was read beforehand: var clientCertificate = new X509Certificate2(iOSApp.Certificate, iOSApp.CertificatePassword); We are using a p12 certificate file with a private password. Again, it works great for an App in Production but not at all for an App in Development. I made use that I use a new p12 developer certificate for the new App when I tested. I also tested with a p12 production certificate. I get the same error. Any help is appreciated.