Pardon me while I get geeky for a moment.
Newsletter tools that report “opens” do not, in reality, know who opened your email (let alone who read it.) The only method possible right now to measure “opens” is to include a tiny invisible image in the email and hope that the recipient will enable their email program’s ability to include images. In Gmail, for instance, images will not be displayed unless you give permission. Many programs, like Microsoft’s ubiquitous Outlook, have a preview pane, allowing a recipient to read your entire email without ever actually opening it and activating the invisible image trigger that notifies the newsletter tool.