under Tools>Options>Receipts you can tick the box that says, "Request a read receipt for all sent messages".
However, (as I understand it), wht this does is include with the email you are sending a request for an EMAIL TO BE SENT BACK TO YOU. The receiver of the email is asked if they want to send a receipt. As a receiver of such an email I always deline to send one, (because I don't want to slow down my dial up connection). Consequently it i snot a reliable method of knowing if your email has been opened/read.