Exchange 2007 has requires SSL for its WebServices, and event for Exchange 2003 some administrators have enabled this requirement on the IIS. If you are dealing with a self-signed certificate on the server and want to use .NET, you will stumble across this error message:

The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel.

or

The remote certificate is invalid according to the validation procedure.

By default, .NET checks whether SSL certificates are signed by a certificate from the Trusted Root Certificate store. To override this behavior, use the System.Net.ServicePointManager.ServerCertificateValidationCallback property:

   1: ServicePointManager.ServerCertificateValidationCallback = RemoteCertificateValidationCallback;

The callback looks like this:

   1: private static bool RemoteCertificateValidationCallback(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)

   2: {

   3:     return true;

   4: }

This will accept all certificates, regardless of why they are invalid. One option here is to display a warning similar to the Internet Explorer one.

Using C# 3.0, this can even be written with less code:

   1: ServicePointManager.ServerCertificateValidationCallback = (sender, certificate, chain, sslPolicyErrors) => true;