Aspose.Network for .NET v3.9.0.0 has released.

In this major release, we supported S/MIME protocol features. Now you can add a digital signature to your email message.

Here is the sample code:

       // load the certificate and associated private key from a file 
       X509Certificate2 certificate = X509Certificate2.LoadPfx("mycer.pfx", "password");

       // create an instance of MailMessage 
       MailMessage message = new MailMessage();

        // set its properties to desired values 
        message.From = "sender@domain.com";
        message.To = recipient@domain.com;
        message.Subject = "This is a signed message";
        message.TextBody = "Hello, world!";
        // attach a digital signature 
        message.AttachSignature(certificate);
        SmtpClient.Send(message);

Download Link:

http://www.aspose.com/community/files/54/utility-components/aspose.network/default.aspx