Originally Posted by DoubleV
... i.e. we would need to know how many people actually opened the email, ...
Well I am unsure of the ethics of such, though of course an opt-in list gives you some scope.
At the end of the day you are going to be hampered by your users choice/setup of mail client, e.g. for those who receive HTML email , you can append unique ID's to server content , e.g. img src="www.blah.com/img/pic.jpg?sid=1234" where pic.jpg is really a server-side script which logs the 'hit' and then outputs the image.
Of course those who can not or will not accept HTML email are another matter , all you can know for sure is whether the email was accepted (e.g. not bounced to your return address) or not, also firewall configuration could hamper such with paranoid clients.
The tracking of forward's etc is feasible if complex, but again , 1 secure client in the chain will break it.
I would suggest that with your stipulation of working with your existing systems that you are well into custom script territory , what tools ? well you know what I would use , but any language that can utilise HTTP or sockets would do.
not much help am I