Their tracking mechamisn can only work if the email client keeps a constant connection with their server (obviously).
I've set up my firewall to deny all connect requests to the internet from my email clients and set up the email clients to deny all requests for receipts.
Now how do they propose to track what I do with your message when my mailprogram can neither reply to them that I've opened your message nor can the message phone home?

As a last resort I can just read it directly on the server, which is completely passive as it just dumps the data stored on the server to my screen in ASCII format including all headers, sending nothing to anybody.
Tough cookie that can track that...