What is Open tracking?
Detecting that an email was opened by embedding an invisible image that loads from a tracking server — increasingly unreliable and a spam signal in cold email.
Open tracking works by adding a 1×1 image to the HTML message; when the recipient's client loads it, the open is recorded. Two problems: it forces the email to be HTML (a stronger spam signal than plain text), and Apple Mail Privacy Protection and Gmail's image proxy now pre-load images, so "opens" happen without a human. Reply rate is the honest metric. If you do track, use a custom tracking domain and filter out scanner bots.