Emails and mailings from marketing companies that spam the mailbox of many users are not just advertising a product. Such emails are able to track whether the user opened the email, when it was opened, and where the user was at that moment.
PixelBlock is a Gmail extension that blocks email tracking attempts used to detect when opening and reading emails. PixelBlock displays the “red eye” icon when it finds and blocks a tracking attempt in an email. The extension is designed to work with the web version of Gmail.
The developers claim that the extension does not store, save, access or transfer any data from your Gmail account or computer. In fact, this extension works locally in your browser and does not interact with remote servers.