I spent alot of time looking for a solution to see if people read emails I sent. As you probably know, 'return receipts' are pretty useless as PC users can simply ignore the request, most web based email and mobile devices don't even support it.
I tried a few and have found:
http://www.readnotify.com/?from=lih398p8p99jr
to be the best and very reasonably priced. They basically embed an invisible graphic in the email. As long as the recipient doesn't specifically turn off images (most don't anymore since so much email depends on the images), then it can track if they read it, when, geographically, where, how many times and if they forwarded it.
This particular solution has a slick plugin (works best with outlook) to make turning it on and off very simple. However, you can use it without a plugin as well by modifying the recipient address.
The main reason this is important is if you are involved in sales, its very valuable to know if a prospect saw your email and is just ignoring you, vs. never read it, ended up in spam filter etc.
Please use the link above or 'mchinsky' in the promotion code field if you sign up later. It gets me some brownie points and extra months of service