A tracking pixel is a tiny image, usually one pixel square and invisible, placed in an HTML email. Each recipient’s copy contains a unique image address. When the email is displayed and the mail app downloads images, the request reaches the sender’s server, which records an open for that person, along with details such as the time and, sometimes, the device and approximate location.
It is the only practical way to track opens, but it measures image loading, not reading. If a mail app blocks images by default, a real reader isn’t counted. If a privacy feature such as Apple Mail Privacy Protection, or a security scanner, downloads images automatically, an open is recorded for someone who never saw the email. Gmail also fetches images through its own proxy, which hides the reader’s IP address and location.
This is why open-based numbers are best used for trends rather than precise counts, and why clicks are a more dependable measure of engagement. Plain-text emails contain no images, so they can’t record opens at all, which is worth remembering when comparing results between plain and designed emails.
Tracking pixels also raise privacy questions. Under data protection laws, open tracking is a form of processing personal data, so your privacy notice should mention that you measure email engagement. Some senders switch off open tracking for sensitive emails, such as health or financial notices, where knowing exactly when someone opened isn’t necessary.
A Hyderabad clinic sends appointment reminders and a monthly health newsletter. It keeps open tracking on the newsletter to see trends, but turns it off for appointment emails, where the information is personal and the open data isn’t needed.
MailNesto records opens with a tracking image and reports them alongside clicks, and you can switch open or click tracking off for a campaign, or for individual messages sent through the transactional API.