I haven’t used the product on my MacOS (yet) as I prefer the web mail version of FastMail. However, in the era of newsletter tracking (especially open rates and time spent on them), this would immediately make them redundant.
If your livelihood depends on tracking pixels for emails, I think it should prompt a rethink of the process.
MailTrackerBlocker is a plugin for the default Mail app built-in to macOS. When you view your email a 🛑 # button will appear to indicate the number of services blockedMailTrackerBlocker for Mail on macOS