Starting with version 2016 R2, Rebex components support SHA-2 family of hash algorithms (includes SHA-256, SHA-384 and SHA-512) on all .NET Compact Framework platforms.
You encountered a certificate that uses SHA-2 algorithm to compute its hash. Since Microsoft do not support .NET CF any more, the latest version of .NET CF supports only SHA-1 algorithms. But we are working on a version of our components that can accept certificates that uses SHA-2 algorithms. Here you can download a beta build of our [Secure Mail Trial version] that supports SHA-2 and give it a try. You still have to validate the certificate yourself, see the code below for an example how to do it. We are also working on our engine that will validate the SHA-2 based certificates on .NET CF automatically.
The simplest way of certificate validation is to check the certificates thumbprint and compare it to a known (and trusted) value. It can be done like this:
Smtp client = new Smtp();
client.ValidatingCertificate += client_ValidatingCertificate;
// continue with the work here
and the validation handler:
static void client_ValidatingCertificate(object sender, SslCertificateValidationEventArgs e)
if (e.Certificate.Thumbprint == "correct thumbprint")
If you have a licenced version of our component or are trying another one than the Rebex Secure Mail, please contact us at firstname.lastname@example.org with this information and we will send you proper beta build of that component (full version and/or different package).