I thought I would find tons of information on the subject but somehow the implementation of email tracking seems to illude me on the web. I understand that there is the so-called email tracking pixel that you can add to an html email. I suppose the src attribute of this image (pixel) would point to a website action or api that would note the opening of the email, etc. However this seems too easy for there to be email services that charge a ton of money to track emails.
I need to implement my own email tracking and am wondering has anyone implemented an email tracking and what are the caveats to it? I am not looking for specific implementations in php, ruby or asp.net, although such would be more than welcomed. I am more looking for the basic principals and approaches to such an implementation.