Buttondown offers opt-in open tracking. When you enable open tracking, Buttondown will inject what the industry lovingly refers to as a "tracking pixel" into the body of your emails.
Let's say you send a basic email to your subscriber list:
When you enable open tracking, Buttondown will inject a tracking pixel into the body of your email (specifically, the end of the body), like this:
When a subscriber opens your email, that tracking pixel will be loaded, and based on that we create a record of the open event.