What is email open rate?
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Email open rate is the percentage of delivered messages that registered at least one open. It's the metric every email marketer tracks first, and it's been the subject of more debate than almost any other metric in email since 2021.
Traditionally, an "open" is recorded when the recipient's email client loads a tracking pixel, a 1x1 transparent image embedded in the message. When that image loads, your ESP gets a request and logs an open event. Open rate = opens divided by delivered messages.
Here's the reliability problem: mailbox providers and email clients increasingly pre-load images on behalf of users, without the user actually opening the email. Apple Mail started doing this at scale with Mail Privacy Protection (MPP) in September 2021. Gmail routes images through its own proxy too. The result is that a significant portion of "opens" in your analytics are phantom opens, triggered by a machine rather than a human.
What this means in practice: open rates are inflated and unreliable as absolute numbers. An open rate of 40% might mean 40% of recipients opened, or it might mean 20% opened and 20% were phantom opens from Apple's servers. You can't tell from the number alone.
Open rate is still useful as a relative metric. If your rate drops sharply from one campaign to the next or trends down over months, that's worth investigating, even if the absolute number isn't trustworthy. Pair it with click rate and conversion rate for a better picture of actual engagement.
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