Tracking e-mails to ensure they are read.

Can anyone recommend an e-mail tracking system that allows me to see when I recipient received and opened a message? I already know about the function in Thunderbird that allows for delivery reports, but the problem with this is that it tells the recipient that the sender has asked for a reciept and gives them the option to send one, rather than simply giving me the information. I am concerned that certain clients (one in particular actually) never send back a return receipt to confirm they have received something, and I am concerned this is deliberate.

Thank you.