I have two friends who have a strange pathology when it comes to email. They initiate emails but seldom reply. I'll get a friendly, chatty email out of the blue after I haven't heard for months, but if I reply to it, never a followup. Usually I'm not the type of person who wants to track an email receipt (as you can do in Outlook with other people on the same Exchange server), but with these two characters, I'd like to know if they're reading what I send them.
In comes the free service SpyPig. I just tested it and it works; it will let you know when the recipient has opened your email. It works the same way as "magic pixels" are used by ad agencies to see how many impressions their ads are getting: you paste in code for an image (which can be invisible if you like) that is hosted by SpyPig. When the reader opens the email and the image is requested from SpyPig, SpyPig sends you an email that your email was opened. It's a bit of a hassle, in that you have to fill out a form for each email you need to send; but when you really need to know if that extra-important email has been read, it's worth it. Of course, if the recipient happens to read it in a text-only email client, it probably won't work, but that's probably rare these days.