Atomic Email Tracker is a web based service for monitoring newsletter efficiency. Using it, you know how many people actually read newsletter, click link in messages, georaphical distribution, etc. More detailed information is available here. Using it, you know how many people actually read newsletter. You can read here how it works.
Usually, Atomic Email Tracker works on our server and you only subscribe to the service. By the way, you may use tracking scripts on your own webserver.