Open tracking
How an open is measured, and why the number is a floor rather than a count
Open tracking works by embedding a tiny transparent image in the HTML of a message. When a mail client loads that image from the server, the request is recorded as an open.
That mechanism explains everything odd about the number.
Why it undercounts
A client that blocks remote images records no open, and blocking is the default in a great many of them. Someone who read your message carefully in a plain-text client is invisible.
Why it also overcounts
Apple Mail Privacy Protection and similar features pre-fetch images for messages the user has not opened, which registers an open that never happened. Corporate security scanners do the same when they follow links and load resources.
So the number is neither a count nor an estimate. It is most useful as a relative measure — this subject line against that one, to the same audience, in the same week — and close to meaningless as an absolute.
Clicks are the sturdier signal. A click needs a deliberate act, and it is not manufactured by a privacy proxy.
In Sendly
Opens and clicks are recorded as events, visible per campaign and available through the API and webhooks. Because an open is a request for an image, tracking requires an HTML body — a plain-text message cannot be tracked, by anyone, ever.
Engagement still matters for deliverability even when you cannot measure it precisely, which is the argument for acting on the trend rather than the figure.
See Tracking.