[Teotwawki]

This is a plugin that has to be installed on the client? Can’t we just not install it?

I'm thinking the client in this case is the email sender who can tell that you, as the recipient have opened it. The recipient can't do anything about it and wouldn't even know that it happened. My concern is that it also shows where you are. Potential to let people know when you are away from your house so the thieves can drop by while you are on vacation.

[Windflier]

I'm thinking the client in this case is the email sender who can tell that you, as the recipient have opened it. The recipient can't do anything about it and wouldn't even know that it happened.

That's exactly how I understand the article. This is a function that operates from the sender's end, and allows them to know what you've done with the mail they've sent you without your express knowledge.

Very creepy stuff.