Most email clients can request a return receipt. One way to set this up in Thunderbird for example is to go to “Preferences > Advanced > General > Return Receipts …” and check the “When sending messages, always request a return receipt”
The problem with this approach is that when the person you’re emailing has set his email client to reject that request (or does that manually every time), you will not get a receipt back. And honestly speaking there is no way to force that. Now there are some services out there that promise to track your mail when that’s read, and those rely on embedding a transparent gif that will call home when the message is opened. I tried a few of these services: the free ones did not deliver (or seemed too shady for my taste) and the paid ones did not look too good either. So I cooked up a quick solution that I can use when needed (like when I’m tracking my brother on his honeymoon trip *evil grin*)
The code is below, pretty self explanatory. You will need a transparent gif/png. That’s easy too 😉
Edit: Looks like some services are not so bad. Checkout bananatag and YesWare Email Tracking