Managed C++/CLI - Encrypt email - S/MIME

The following Managed C++/CLI example codes demonstrate how to encrypt email to S/MIME format with digital signature.

Email Encryption

After the recipient received your email with digital signature, the recipient can get your digital certificate public key from your digital signature. Then the recipient can encrypt an email with your public key and send it to you. Only you can decrypt this email with your private key. That is how S/MIME can protect your email content. If you don’t expose your digital certificate private key to others, none can read your email which is encrypted by your public key.

If you received an email with digital signature, your email client usually stores the public key of the sender in “Control Panel” -> “Internet Options” -> “Content” -> “Certificates” -> “Other People”.

Then you can use the following code to encrypt email and send it to your recipient.


Before you can use the following codes, please download EASendMail SMTP Component and install it on your machine at first. Full sample proejcts are included in this installer.

Install from NuGet

You can also install the run-time assembly by NuGet. Run the following command in the NuGet Package Manager Console:

Install-Package EASendMail


If you install it by NuGet, no sample projects are installed, only .NET assembly is installed.

Add reference

To use EASendMail SMTP Component in your project, the first step is Add reference of EASendMail to your project. Please create or open your project with Visual Studio, then go to menu -> Project -> Add Reference -> .NET -> Browse..., and select Installation Path\Lib\net[version]\EASendMail.dll from your disk, click Open -> OK, the reference of EASendMail will be added to your project, and you can start to use it to send email in your project.

add reference in c#/

.NET assembly

Because EASendMail has separate builds for .Net Framework, please refer to the following table and choose the correct dll.

Separate builds of run-time assembly for .NET Framework 1.1, 2.0, 3.5, 4.0, 4.5, 4.6.1, .NET Core 3.1, .NET 5.0, .NET Standard 2.0 and .NET Compact Framework 2.0, 3.5.

File .NET Framework Version
Lib\net20\EASendMail.dll Built with .NET Framework 2.0
It requires .NET Framework 2.0, 3.5 or later version.
Lib\net40\EASendMail.dll Built with .NET Framework 4.0
It requires .NET Framework 4.0 or later version.
Lib\net45\EASendMail.dll Built with .NET Framework 4.5
It requires .NET Framework 4.5 or later version.
Lib\net461\EASendMail.dll Built with .NET Framework 4.6.1
It requires .NET Framework 4.6.1 or later version.
Lib\netcoreapp3.1\EASendMail.dll Built with .NET Core 3.1
It requires .NET Core 3.1 or later version.
Lib\net5.0\EASendMail.dll Built with .NET 5.0
It requires .NET 5.0 or later version.
Lib\net6.0\EASendMail.dll Built with .NET 6.0
It requires .NET 6.0 or later version.
Lib\netstandard2.0\EASendMail.dll Built with .NET Standard 2.0
It requires .NET Standard 2.0 or later version.
Lib\net20-cf\EASendMail.dll Built with .NET Compact Framework 2.0
It requires .NET Compact Framework 2.0, 3.5 or later version.
Lib\net35-cf\EASendMail.dll Built with .NET Compact Framework 3.5
It requires .NET Compact Framework 3.5 or later version.

C++/CLI - Encrypt email - S/MIME - example

The following example codes demonstrate encrypting email message with digital signature - S/MIME. In order to run it correctly, please change SMTP server, user, password, sender, recipient value to yours.


To get full sample projects, please download and install EASendMail on your machine.

#include "stdafx.h"

using namespace System;
using namespace System::Security::Cryptography::X509Certificates;
using namespace EASendMail;

X509Certificate2^ _findCertificate(String ^storeName, String ^emailAddress)
    X509Certificate2 ^cert = nullptr;

    X509Store ^store = gcnew X509Store(storeName, StoreLocation::CurrentUser);

    X509Certificate2Collection ^certfiicates = store->Certificates->Find(
        X509FindType::FindBySubjectName, emailAddress, true);

    if (certfiicates->Count > 0)
        cert = certfiicates[0];

    return cert;

int main(array<System::String ^> ^args)
        SmtpMail ^oMail = gcnew SmtpMail("TryIt");

        // Set sender email address, please change it to yours
        oMail->From = "";

        // Set recipient email address, please change it to yours
        oMail->To = gcnew AddressCollection("");

        // Set email subject
        oMail->Subject = "test encrypted email from Managed C++/CLI";

        // Set email body
        oMail->TextBody = "this is a test encrypted email sent from Managed C++";

        X509Certificate2 ^signerCertificate = _findCertificate("My", oMail->From->Address);
        if (signerCertificate == nullptr)
            throw gcnew Exception("No signer certificate found!");

        oMail->From->Certificate2 = signerCertificate;

        // You can also load the signer certificate from a pfx file.
        /* String ^pfxPath = "D:\\TestCerts\\signer.pfx";
        X509Certificate2 ^signerCertFromPfx = gcnew X509Certificate2(pfxPath,
            X509KeyStorageFlags::Exportable | X509KeyStorageFlags::UserKeySet);

        oMail->From->Certificate2 = signerCertFromPfx;
        // If you use it in web application,
        // please use  X509KeyStorageFlags::Exportable | X509KeyStorageFlags::MachineKeySet

        // If you use it in .NET core application
        // please use X509KeyStorageFlags::Exportable | X509KeyStorageFlags::EphemeralKeySet

        int count = oMail->To->Count;
        for (int i = 0; i < count; i++)
            MailAddress ^oAddress = dynamic_cast<MailAddress^>(oMail->To[i]);

            X509Certificate2 ^encryptCert = _findCertificate("AddressBook", oAddress->Address);
            if (encryptCert == nullptr)
                encryptCert = _findCertificate("My", oAddress->Address);

            if (encryptCert == nullptr)
                throw gcnew Exception("No encryption certificate found!");

            oAddress->Certificate2 = encryptCert;

            // You can also load the encryptor certificate from a cer file like this
            String ^cerPath = "D:\\TestCerts\\encryptor.cer";
            X509Certificate2 ^encryptCertFromFile = gcnew X509Certificate2(cerPath);
            oAddress->Certificate2 = encryptCertFromFile;

        // Your SMTP server address
        SmtpServer ^oServer = gcnew SmtpServer("");

        // User and password for ESMTP authentication, if your server doesn't require
        // User authentication, please remove the following codes.
        oServer->User = "";
        oServer->Password = "testpassword";

        // Most mordern SMTP servers require SSL/TLS connection now.
        // ConnectTryTLS means if server supports SSL/TLS, SSL/TLS will be used automatically.
        oServer->ConnectType = SmtpConnectType::ConnectTryTLS;

        // If your SMTP server uses 587 port
        // oServer->Port = 587;

        // If your SMTP server requires SSL/TLS connection on 25/587/465 port
        // oServer->Port = 25; // 25 or 587 or 465
        // oServer->ConnectType = SmtpConnectType::ConnectSSLAuto;

        Console::WriteLine("start to send encrypted email ...");

        SmtpClient ^oSmtp = gcnew SmtpClient();
        oSmtp->SendMail(oServer, oMail);

        Console::WriteLine("email was sent successfully!");
    catch (Exception ^ep)
        Console::WriteLine("failed to send email with the following error:");

    return 0;

Encryption algorithm

You can use SmtpMail.EncryptionAlgorithm property to set RC2, RC4, 3DES, AES128, AES192 or AES256 encryption algorithm. RSAES-OAEP (AES128, AES192 and AES256) is recommended.

RSA-OAEP Encryption with SHA256 HASH

If you need to use RSA-OAEP encryption with sha256 scheme, please have a look at this topic:

RSASSA-PSS + RSA-OAEP Encryption with SHA256

TLS 1.2 protocol

TLS is the successor of SSL, more and more SMTP servers require TLS 1.2 encryption now.

If your operating system is Windows XP/Vista/Windows 7/Windows 2003/2008/2008 R2/2012/2012 R2, you need to enable TLS 1.2 protocol in your operating system like this:

Enable TLS 1.2 on Windows XP/Vista/7/10/Windows 2008/2008 R2/2012



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