public MessagingHandler( 
   Message message,
   RecipientCollection recipients,
   CallbackHandler callbackHandler 
)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Dynamicweb.Examples.CSharp.EmailMessaging
{
    class UsingCallbackHandlerSample
    {
        public void SendWithCallbackHandler()
        {
            //Create a recipient collection and add recipient
            var recipients = new Dynamicweb.EmailMessaging.RecipientCollection();
            var recipient = new Dynamicweb.EmailMessaging.Recipient();
            recipient.Name = "Test";
            recipient.EmailAddress = "test12@testdomain.tld";
            recipients.Add(recipient);

            //Get a message with a specific id.
            var message = Dynamicweb.EmailMessaging.Message.GetMessageById(20);

            //Instanciate a new message handler.
            //This will use the TestCallbackHandler, which will alter the process.
            var handler = new Dynamicweb.EmailMessaging.MessagingHandler(message, recipients, new TestCallbackHandler());

            //Start the send process.
            var processStarted = handler.Process();

            if (!processStarted)
                throw new Exception("Unable to start process");
        }

    }

    class TestCallbackHandler : Dynamicweb.EmailMessaging.CallbackHandler
    {
        //This is one of the methods you can override in the CallbackHandler class.
        public override void OnBeforeDelivery(Dynamicweb.EmailMessaging.CallbackHandlerContext context, Dynamicweb.EmailMessaging.Message message, Dynamicweb.EmailMessaging.Recipient recipient, Dynamicweb.EmailMessaging.MessageDeliverer deliverer, System.Net.Mail.MailMessage mailMessage)
        {
            //Imagine you want to change the way a message is delivered for a specific message and recipient.

            //Do the check
            if (message.Id == 20 && recipient.EmailAddress == "test12@testdomain.tld")
            {
                //Set the new delivery provider
                deliverer.DeliveryProvider = new TestDeliveryProvider();
            }
        }
    }
    class TestDeliveryProvider : Dynamicweb.EmailMessaging.MessageDeliveryProvider
    {
        public override bool Deliver(System.Net.Mail.MailMessage mailMessage)
        {
            //This is just a simple implementation using the Dynamicweb EmailHandler system.
            //This could also be an external system that can send an System.Net.Mail.MailMessage.
            return Dynamicweb.EmailHandler.Send(mailMessage);
        }
    }
}