(I posted the same question in dotnet.security since the problem is
cross-domain: SSL and Exchange 2003 protocol requirements.)
In order to automate some mail processing I need to access POP3 servers over
a
secure channel. A couple of MSDN articles recently written by Dominick Baier
was helpful to me when I implemented a solution in C# with the .NET Framework
2.0. It works fine as far as I don't try to connect to an MS Exchange 2003
server: In this case, the connection is terminated by the server. Here is the
somewhat simplified code:

class CMyPOPClient
{
 TcpClient _Channel = null;
 NetworkStream _Stream = null;
 SslStream _SSLStream = null;
 .
 .
 .
 void secureConnect(String strHost, Int32 iPort)
 {
   .
   .
   .
   _Channel = new TcpClient());
   _Channel.Connect(strHost, iPort);
   _Stream = _Channel.GetStream();
   _SSLStream = new SslStream(
     _Channel.GetStream(),
     false,
     new RemoteCertificateValidationCallback(validateServerCertificate),
     null);
   _SSLStream.AuthenticateAsClient( // <== here a
System.Net.Sockets.SocketException
                                    // is thrown with error code 10053
     strHost,
     null,
     SslProtocols.Tls,
     false);
   .
   .
   .
 }
 .
 .
 .
}

I should mention that the code is running on a WinXPSP2 PC which is in the
same Microsoft domain as the Win 2003 Server on which MS Exchange is running.

Actually I don't need my code being capable to connect to MS Exchange 2003
but I don't like to see it fail. Has anybody an idea what is expected by MS
Exchange 2003?