This is a plugin for macOS Apple Mail which seeks to block the tracking pixels sent in many emails, especially commercial emails. Tracking pixels have become increasingly prevalent over the past few years, and they can report everything from your physical IP address to the kind of device you’re on.
I’m currently on a trial using the email service Hey, which blocks tracking pixels on their end. I’m finding the service as a whole to be superb and the tracking pixel blocking is a nice benefit to have. However, not everyone is looking to potentially shell out $100/yr ($8.33/mo) on an email service, so plugins like MailTrackerBlocker are a nice alternative.
Unfortunately, this is only for macOS, so those who use Apple Mail or any other client on iOS/iPadOS are still at risk of being tracked if you check your email on your phone. Hopefully, this issue has rung enough alarms in Cupertino to make tracking pixel blocking a feature through all platforms.