Please see the answers to these questions:
Is there any way to track whether an email has been opened?
Quote from the answer in the first question:
Mail clients block pretty much all of these kinds of attempts. The best idea is to give them an image that they would want to see if they read the message, and therefore they choose to display images in their mail client.
So yeah, that's pretty much the best advice, and even then it may not work well if the email client is caching your images (though it should still be able to log the first download). There's no foolproof method of actually verifying if someone's read your email, unless you can make them visit your website with a link in the email.