static void Main(string[] args)
    {
        var cert = new X509Certificate2(
          @"sender.pfx",
          "password",
          X509KeyStorageFlags.MachineKeySet);

        var signer = new CmsSigner(cert)
        {
            IncludeOption = X509IncludeOption.EndCertOnly
        };

        Message message = new Message();
        message.From = new Address("sender@gmail.com");
        message.To.Add("me@gmail.com");
        message.Subject = "Subject";
        message.BodyHtml.Text = "Html";
        message.BodyText.Text = "Plain Text";
        message.BuildMimePartTree();
        message.SmimeAttachSignatureBy(signer);

        SmtpClient.SendSsl(
            message,
            "smtp.gmail.com",
            465,
            "sender@gmail.com",
            "password",
            SaslMechanism.Login);
    }