You are talking about a "mail beacon", frequently used by spammers to validate that messages that they send off are being received by the people that they spam them out to.
You could use a similar technique. Setup a web server somewhere with full logging enabled. Create a HTML e-mail with some links to a page on this server in an e-mail and send it off. Whenever anyone reads the message, you'll know it, since it will get a log file entry for that page.
Note that you need to make a unique HTML page for each e-mail so you can track it. Also, if someone opens the same message more than one time on their computer it will probably show up as a "hit" on your web server. You just have to keep track of IP addresses and try to see what the difference is.
Note that I haven't actually done this, it's just theory, but should be sound.
- G