Open tracking works by loading a tiny image. That makes it wrong in both directions.
Under-counting
Clients that block images never register the open, even though the person read the email.
Over-counting
Privacy proxies and security scanners fetch images automatically, registering opens nobody made.
What to do
Optimise on replies. Use open rate only for coarse comparison between two variants sent under identical conditions — never as a success metric on its own.
Important notes
Tracking also adds a redirect to every link, which very slightly costs deliverability. For a first cold email, sending without tracking is a defensible choice.
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