System.Security.Authentication.AuthenticationException

The token supplied to the function is invalid

Full exception looks like this:

System.Security.Authentication.AuthenticationException : A call to SSPI failed, see inner exception.
----> System.ComponentModel.Win32Exception : The token supplied to the function is invalid

Most likely your client tries to use TLS 1.2 but you are using old certificate on the server (e.g. signed using md5RSA algorithm).

There are 2 options for you:

1. Regenerate the certificate (especially if it’s self-signed).
2. Use older TLS version (TLS 1.1). You can force Mail.dll or Ftp.dll to use TLS 1.1 using the following code:

   using (XXX client = new XXX())
   {
       client.SSLConfiguration.EnabledSslProtocols = SslProtocols.Tls11;
   
       client.ConnectSSL("host");
   
       client.Close();
   }
   
   

   Please contact your server administrator as TLS 1.1 and SSL 3.0 aren’t considered secure anymore.

The client and server cannot communicate, because they do not possess a common algorithm

Full exception looks like this:

System.Security.Authentication.AuthenticationException : A call to SSPI failed, see inner exception.
----> System.ComponentModel.Win32Exception : The client and server cannot communicate, because they do not possess a common algorithm

There are 2 possible scenarios:

1. In most cases this means that the client is trying to use older SSL protocols like SSL 3.0, TLS 1.0 or TLS 1.1, but the remote server requires modern protocol – TLS 1.2.

   By default all our clients support TLS 1.2. Some older versions need to be told to use TLS 1.2, it is also a good practice to force TLS 1.2 only:

   using (XXX client = new XXX())
   {
       client.SSLConfiguration.EnabledSslProtocols = SslProtocols.Tls12;
   
       client.ConnectSSL("host");
   
       client.Close();
   }
   

2. Second option is the server is not supporting TLS 1.2 – you’ll need to use TLS 1.1:

   using (XXX client = new XXX())
   {
       client.SSLConfiguration.EnabledSslProtocols = SslProtocols.Tls11;
       // client.SSLConfiguration.EnabledSslProtocols = SslProtocols.Ssl3;
   
       client.ConnectSSL("host");
   
       client.Close();
   }
   

Please contact your server administrator as TLS 1.1 and SSL 3.0 aren’t considered secure anymore.