With email spam, oftentimes each email has a slightly modified link all redirected to the same inviisible 1x1 image.
What kind of email client will actually go and fetch that image from a remote URL?
Maybe M$ Outlook will... but I would be suprised if that was the default configuration
nowadays. This is absolutely the oldest trick in the book for infecting email software... include a malformed jped/pdf/whatever in the email.
However, I recall there was a company making money out of a service whereby you included the 1-pixel image in your outgoing email(s) and that way you could tell when the recipient has read it. Without some trick like that, there is no way to tell if somebody has opened an email.