UnseenMail, an extension that blocks email trackers.
A simple extension that currently blocks the following list of trackers:
I will work on making a more general solution for such a thing. Also I might work more on the UI to improve it.
An email tracker is an extension that informs you whenever your email has been opened by the receiver.
An email tracker blocker is an extension that blocks "Email Trackers", So if you open your email, the sender wouldn't get a notification of you opening it.