C / C++ / MFC

Digital signature prevents email content is faked or changed in transport level. Encrypting email protects email content from exposure to inappropriate recipients. Both digital signature and email encrypting depend on digital certificate.

If you have an email digital signature certificate installed on your machine, you can find it in Control Panel-interner option-content-Certificates-Personal
Then you can use your email certificate to sign the email by the following code. If you don't have a certificate for your email address, you MUST get a digital certificate for personal email protection from third-party certificate authorities such as www.verisign.com.
#include "stdafx.h"

#include "easendmailobj.tlh"
using namespace EASendMailObjLib;

int _tmain(int argc, _TCHAR* argv[])
{
    ::CoInitialize( NULL );

    IMailPtr oSmtp = NULL;
    oSmtp.CreateInstance( "EASendMailObj.Mail");
    oSmtp->LicenseCode = _T("TryIt");
    
    // Set your sender email address
    oSmtp->FromAddr = _T("test@emailarchitect.net");
    
    // Add recipient email address
    oSmtp->AddRecipientEx( _T("support@emailarchitect.net"), 0 );
    
    // Set email subject
    oSmtp->Subject = _T("email from Visual C++ with digital signature(S/MIME)");
    
    // Set email body
    oSmtp->BodyText = _T("this is a test email sent from Visual C++ with digital signature");
    
    // Your SMTP server address
    oSmtp->ServerAddr = _T("smtp.emailarchitect.net");
    
    // User and password for ESMTP authentication, if your server doesn't 
    // require User authentication, please remove the following codes.
    oSmtp->UserName = _T("test@emailarchitect.net");
    oSmtp->Password = _T("testpassword");

    // If your SMTP server requires SSL connection, please add this line
    //oSmtp->SSL_init();

    // Add signer digital signature
    if( oSmtp->SignerCert->FindSubject(_T("test@emailarchitect.net"), 
        CERT_SYSTEM_STORE_CURRENT_USER , _T("my")) == VARIANT_FALSE )
    {
        _tprintf(_T("Error with signer certificate; %s\r\n"), 
            (const TCHAR*)oSmtp->SignerCert->GetLastError());
        return 0;
    }

    if( oSmtp->SignerCert->HasPrivateKey == VARIANT_FALSE )
    {
        _tprintf(_T("certificate does not have a private key, it can not sign email.\r\n" )); 
        return 0;			
    }

    _tprintf(_T("Start to send email ...\r\n" ));
  
    if( oSmtp->SendMail() == 0 )
    {
        _tprintf( _T("email was sent successfully!\r\n"));
    }
    else
    {
        _tprintf( _T("failed to send email with the following error: %s\r\n"),
            (const TCHAR*)oSmtp->GetLastErrDescription());
    }

    if( oSmtp != NULL )
        oSmtp.Release();

    return 0;
}

LibTomMath is hereby released into the Public Domain.  

-- Tom St Denis