using System;

using System.Collections.Generic;

using System.IO;

using System.Security.Cryptography;

using MimeKit;

using MailKit.Net.Smtp;

using MailKit.Security;

​

class Program

{

    static void Main(string[] args)

    {

        var message = new MimeMessage();

        message.From.Add(new MailboxAddress("Sender Name", "sender@example.com"));

        message.To.Add(new MailboxAddress("Recipient Name", "recipient@example.com"));

        message.Subject = "Double DKIM Signed Email Test";

        message.Date = DateTimeOffset.Now;

​

        message.Body = new TextPart("plain")

        {

            Text = "This is a test email with double DKIM signing."

        };

​

        // Load the private key (assuming it's in PEM format)

        string privateKeyPem = File.ReadAllText("private_key.pem");

        RSA rsa = RSA.Create();

        rsa.ImportFromPem(privateKeyPem);

​

        // Headers to be signed

        var headersToSign = new HashSet<string> { "From", "To", "Subject", "Date" };

​

        // Sign with the sending domain

        var sendingDomainSigner = new DkimSigner(rsa, "example.com", "selector1")

        {

            HeaderCanonicalizationAlgorithm = DkimCanonicalizationAlgorithm.Relaxed,

            BodyCanonicalizationAlgorithm = DkimCanonicalizationAlgorithm.Simple,

            AgentOrUserIdentifier = "@example.com",

            QueryMethod = "dns/txt",

            SignatureAlgorithm = DkimSignatureAlgorithm.RsaSha256,

            Headers = headersToSign

        };

        sendingDomainSigner.Sign(message);

​

        // Sign with the mail server domain

        var serverDomainSigner = new DkimSigner(rsa, "mailserver.com", "selector2")

        {

            HeaderCanonicalizationAlgorithm = DkimCanonicalizationAlgorithm.Relaxed,

            BodyCanonicalizationAlgorithm = DkimCanonicalizationAlgorithm.Simple,

            AgentOrUserIdentifier = "@mailserver.com",

            QueryMethod = "dns/txt",

            SignatureAlgorithm = DkimSignatureAlgorithm.RsaSha256,

            Headers = headersToSign

        };

        serverDomainSigner.Sign(message);

​

        using (var client = new SmtpClient())

        {

            client.Connect("smtp.example.com", 587, SecureSocketOptions.StartTls);

            client.Authenticate("username", "password");

            client.Send(message);

            client.Disconnect(true);

        }

​

        Console.WriteLine("Email sent successfully with double DKIM signing, including key headers.");

    }

}