Managed C++/CLI - DomainKeys and DKIM Signature

The following Managed C++/CLI example codes demonstrate how to add DomainKeys and DKIM signature to email and send it using SMTP server.

DKIM and DomainKeys

DomainKeys is a technology proposal that can bring black and white back to this decision process by giving email providers a mechanism for verifying both the domain of each email sender and the integrity of the messages sent. To learn more detail about DomainKeys, please refer to

DKIM is a similar technology as Domainkeys. To learn more detail about DKIM, please refer to RFC4871.

Before you can add DomainKeys and DKIM signature to email, you need to generate the certificate and X-DK-File like this:

Download Certificate for your DomainKeys/DKIM key pair

Please click here to go to this online tool at first. Then input your email sender domain in it and click “Download Certificate” and save the downloaded pfx file to your local disk. This file contains your DomainKeys and DKIM key pair.

To sign the DomainKeys with EASendMail, we also need to create a text file with notepad and save it.

For example: We create a emailarchitectdomainkeys.txt and contains the following content:

dk_s: s1024
PFXPath: c:\my cert\emailarchitect.pfx
PFXPassword: mypassword
DKIM: yes
DomainKey: yes

// dk_d is the email sender domain,
// dk_s is the domain public key selector.
// PFXPath is the PFX file full path.
// PFXPassword is the PFX file password.

We also need to deploy the certificate Public Key to your domain DNS server. You can export the public key by the online tool:

Deploy Public Key

Once you get the public key, you should set a TXT record in your domain DNS server. For example: your selector is s1024, your domain is, and then you should create a TXT record for and set the following content in the record.      text =

        "t=y; k=rsa; p=MIGfMA0GCSqGS...hSinuQIDAQAB"

To learn more detail about Public Key deployment, please refer to


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 2.1 and .NET Compact Framework 2.0, 3.5.

File .NET Framework Version
Lib\net11\EASendMail.dll Built with .NET Framework 1.1
It requires .NET Framework 1.1, 2.0, 3.5 or later version.
Lib\net20\EASendMail.dll Built with .NET Framework 2.0
It requires .NET Framework 2.0, 3.5 or later version.
Lib\net35\EASendMail.dll Built with .NET Framework 3.5
It requires .NET Framework 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\netcoreapp2.1\EASendMail.dll Built with .NET Core 2.1
It requires .NET Core 2.1 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 - DomainKeys and DKIM Signature - Example

The following example codes demonstrate sending email with DomainKeys and DKIM signature. 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 EASendMail;

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 = "";

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

        // Set email body
        oMail->TextBody = "this is a test email sent from Managed C++ project, do not reply";

        // Add DomainKeys and DKIM configuration file
        // Then this email will be signed automatically
        oMail->Headers->ReplaceHeader("X-DK-File", "c:\\emailarchitectdomainkeys.txt");

        // 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 email from Managed C++...");

        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;


If your SMTP server has implemented DKIM, you don’t have to sign DKIM in your codes, your email will be signed by your server automatically.

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



If you have any comments or questions about above example codes, please click here to add your comments.