Open pixel
An open pixel, also called a tracking pixel, is a tiny, transparent image, typically 1 pixel by 1 pixel, embedded invisibly in a marketing email that registers an open when it is loaded from a remote server.
When a recipient opens an email, and their client loads images, the pixel image is requested from the server that hosts it. That request is logged, recording that the email was opened, along with data such as the time, approximate location, and device type associated with the request.
Open pixels have been the standard mechanism for email open tracking for decades, but their reliability has been significantly affected by privacy changes over the past few years. Apple’s Mail Privacy Protection (MPP), introduced in 2021, pre-loads email content, including tracking pixels, on Apple’s own servers before delivering the email to the recipient’s device. This means a pixel load is recorded even if the recipient never actually opens the email, inflating open rates in Apple Mail substantially and making raw open rate data increasingly unreliable as a measure of genuine human engagement.
For email marketers, the practical implication is that open rate should no longer be treated as a primary performance metric. Click rate, reply rate, and conversion rate are more reliable indicators of genuine engagement because they require deliberate human action that pre-loading cannot replicate. Open data still has value for directional trend analysis and for identifying completely unengaged contacts, but interpreting it as a precise measure of how many people read your email is no longer valid in the post-MPP landscape.
How does an open pixel work technically?
An open pixel is a 1×1 transparent GIF or PNG image embedded in the email’s HTML with a unique URL that identifies the specific email and recipient. When the email is opened, and images are loaded, the email client requests that image from the hosting server. The server logs the request and records an open event against that contact’s record. The pixel itself is invisible to the recipient because it is transparent and too small to see. Most email marketing platforms insert the tracking pixel automatically into every outgoing email.
What is Apple Mail Privacy Protection, and how does it affect open tracking?
Apple Mail Privacy Protection (MPP), launched in September 2021 for iOS 15 and macOS Monterey, allows Apple Mail users to opt in to a feature that pre-fetches email content, including images and tracking pixels, through Apple’s proxy servers before the email is delivered to the device. This means the pixel fires, and an open is recorded on Apple’s server regardless of whether the recipient actually opens the email. Because Apple Mail has a large share of email client usage, particularly among business users, this has significantly inflated reported open rates for many senders.
Should I still track email opens if open data is unreliable?
Yes, with appropriate caveats. Open data still provides useful signals for identifying contacts who have definitely never engaged (contacts who show zero opens over a long period, even accounting for MPP inflation, are genuinely disengaged), for directional campaign comparisons, and for triggering certain automation workflows where a rough signal of activity is sufficient. What has changed is that open rate should not be used as a primary KPI, reported to leadership as a precise measure of readership, or used as the sole criterion for lead qualification or list segmentation. Complement it with click rate and conversion data for a more accurate picture.