Ugly Email for Chrome and Firefox will alert you to possible email trackers before even opening the message.
Ugly Email will allow you to keep your email habits private by identifying and blocking email trackers. It scans through your inbox, looking for emails containing tracking pixels. Any email found with trackers is labeled with an eyeball icon, and the tracking pixel is blocked. Keep in mind, this is not foolproof, some may get by.
All of the Ugly Email's data is kept locally on your browser's IndexDB storage. They do not track, transfer, or store any of your information.
Ugly Email is a useful extension/add-on that takes away an important tool used by email marketers that provides them insight into your email reading habits and puts it back into your control.