Here is an article describing a method being used to alert the sender that their email (which they had sent to you) is now being opened by you. Huh? Yep -- read on. How to prevent this, you ask? Well, not easy to prevent, it seems. As an example, since I use Gmail via their web service, there is one setting that I -- or you -- could make that might help: tell Gmail to ask you before opening an image. Is it worth the extra click and decision by you? This doesn't completely block the electronic snoop, as there are other ways to electronically snoop emails.
Don't lose any sleep over this. Just something to be aware of.