Trocker

Trocker does one job: it stops senders from learning when you open their emails. Many newsletters and sales emails hide a tiny image that loads from the sender's server, and that request tells them you opened it. Trocker blocks those images, shows you which messages were tracked and tries to bypass links that log your clicks.

It works in the background on any site and adds extra features in Gmail, Yahoo Mail and Outlook.com. It matches known tracker addresses and uses a heuristic that treats very small images as trackers, since a sender who wants you to see an image will make it larger than one pixel. In Gmail and Yahoo, images load through a proxy, so it can block them. Outlook.com has no proxy, so there Trocker can expose a tracker but not always block it.

The trade-offs are honesty about reach and a small project. It needs access to all websites to catch trackers and bypass links, though the code is public on GitHub under Apache 2.0, and the developers say it makes no outbound network requests. A heuristic can miss trackers that avoid tiny images. If you want the free, open-source route to blocking email tracking, this is the one to install.