Thunderbird displays the message, "To protect your privacy, Thunderbird has blocked remote content in this message." Just how does blocking remote content protect my privacy? I searched the support website for Thunderbird and couldn't find anything.
It is possible to send an HTML E-mail that loads an image from a remote Web server. The reference to the image could be something other than an image, even though an image is actually displayed in the E-mail. This could contain variables that I want to learn, such as who received my E-mail, and many other variables gathered when the E-mail client loads the "image" from the Web server. And there are many more techniques.