Open rate estimates how many recipients opened an email, but privacy features now inflate it.
Key points
- Opens are detected by a tracking pixel, a form of web beacon that loads from the sender's server [1].
- Apple's Mail Privacy Protection, introduced in 2021, stops senders from seeing when a message is opened [2][3], and its image preloading records opens that may never have happened [5].
- Mailchimp defines open rate as unique opens divided by delivered emails and flags the effect of privacy features on the numbers [4].
- For cold email, Reply Rate and Positive Reply Rate are better success measures; Open Tracking can also add markup that looks like bulk mail.
- Open rate still helps as a rough health check: a sudden drop can hint at Inbox Placement problems.
How open rate is measured
When an HTML email is sent with tracking, it includes a tiny, invisible image unique to the recipient. When the email client loads the image, the sender's server logs an open. This technique is a type of web beacon [1]. Open rate is then unique opens divided by delivered emails. Mailchimp's help pages describe the calculation and note that opens are estimates, because image loading depends on the client and settings [4]. Plain-text emails cannot be tracked this way at all, since there is no image to load. Some clients block images by default, which undercounts opens, while others fetch them automatically, which overcounts them.
The privacy problem
Apple announced Mail Privacy Protection in June 2021 and shipped it with iOS 15 and macOS Monterey. It masks the recipient's IP address and stops senders from using invisible pixels to learn when a message is opened [2]. Apple's current support guide describes the same protection in the Mail app [3]. In practice, Apple routes images, including tracking pixels, through a proxy that preloads them at unpredictable times, so a message can register as opened even if nobody read it [5]. Because a large share of people read email in Apple Mail, many opens recorded since then are machine opens rather than human ones. Other providers proxy or cache images too. The result is that open rates look higher than real engagement and vary for reasons unrelated to the email. Comparing opens across time periods or tools is especially unreliable.
What to use instead
For Cold Email, judge success by replies. Reply Rate shows whether people engaged, and Positive Reply Rate shows whether they were interested. For deliverability, use the data mailbox providers publish, such as spam rate and domain reputation in Google Postmaster Tools, rather than inferring inbox placement from opens. Some teams turn open tracking off entirely for cold outreach, because a tracking pixel and its domain add markup that makes a one-to-one email look more like a campaign. If you keep it, treat opens as a rough directional signal, and never use an open as the trigger for a follow-up that implies the recipient read the email.
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