<NotificationHandlerAssembly>MySharepointHandler, Version=1.0.0.0, Culture=neutral, PublicKeyToken=abcdabcdabcd</NotificationHandlerAssembly>
  <NotificationHandlerClassName>MySharepointHandler.MyNotifyHandler</NotificationHandlerClassName>

public class MyNotifyHandler : IAlertNotifyHandler
{
    public bool OnNotification(SPAlertHandlerParams ahp)
    {
        using (new SPMonitoredScope("MySharepointHandler.MyNotifyHandler OnNotification"))
        {
            using (SPSite site = new SPSite(ahp.siteUrl + ahp.webUrl))
            {
                using (SPWeb web = site.OpenWeb())
                {
                    string to = ahp.headers["to"];

                    if (to.IndexOf("@mydomain.com") < 0)
                    {
                        string NewBody = ahp.body;
                        NewBody = NewBody.Replace(InternalURL, ExternalURL);
                        SPUtility.SendEmail(web, ahp.headers, NewBody);
                    }
                    else
                    {
                        SPUtility.SendEmail(web, ahp.headers, ahp.body);
                    }
                    return true;
                }
            }
        }
    }
}