Open Tracking

Open tracking is a method of recording when an email is opened by embedding a tiny, uniquely addressed image, often called a tracking pixel, that loads from the sender's server. Privacy features such as Apple Mail Privacy Protection and image proxies have made the data unreliable.

Deliverability & Email InfrastructureUpdated September 30, 2026

In short

Open tracking uses a hidden image to guess when mail is opened, and the signal is now noisy.

Key points

  1. A tracking pixel is a form of web beacon: a small remote image whose URL identifies the recipient and message [1].
  2. Apple's Mail Privacy Protection, introduced in 2021, stops senders from using invisible pixels to learn when a message is opened and hides the reader's IP address [2].
  3. Gmail scans images before they are shown and lets users turn off automatic image loading, which also affects pixel data [3].
  4. Because of these changes, Open Rate figures can be inflated by automatic loads or deflated by blocked images [1][5].
  5. Tracking pixels and rewritten links often run through a Custom Tracking Domain; a shared tracking domain with a poor reputation can hurt Inbox Placement [4].

How open tracking works

When a sender enables open tracking, the sending system inserts an HTML image tag near the end of the message. The image is usually one transparent pixel, and its URL contains a token tied to that specific recipient and send [1]. If the recipient's mail client downloads remote images when the message is displayed, the request reaches the tracking server, which logs an open along with a timestamp and sometimes the IP address and device. The method only works on HTML email; a Plain-Text Email has nothing to load. Link click tracking, the basis of Click-Through Rate (CTR), works in a similar way by rewriting each link to pass through a redirect that records the click. SendGrid's documentation, for example, describes serving click-tracked links, opens and images from a tracking domain, either the provider's shared one or the sender's own [4].

Why open data became unreliable

Several changes broke the link between a pixel load and a human reading the message. Apple's Mail Privacy Protection, announced in 2021 for iOS 15 and macOS Monterey, prevents senders from using invisible pixels to see when mail is opened and masks the user's IP address [2]; in practice, Apple's proxy preloads images, including tracking pixels, so many messages register as opened whether or not they were read [5]. Gmail scans images for suspicious content and gives users control over whether images load [3]. Corporate security filters also fetch links and images to scan them, adding false opens. At the same time, clients with images turned off record no opens at all. The result is that Open Rate is a weak measure of attention, especially for Cold Email.

Should you use open tracking for outbound

Many outbound teams now turn open tracking off for first-touch emails. The pixel adds HTML and an external image to what would otherwise be a simple personal message, and tracking links on a shared domain tie your results to a domain you do not control; filters weigh the reputation of link domains, so a shared one can trigger a Spam Filter or drag on Domain Reputation [4]. The data it provides is also noisy. Reply Rate and Positive Reply Rate are more reliable measures because a reply is an unambiguous signal of attention. If you do track opens, use it to compare broad trends rather than to trigger follow-ups on a single open, and prefer a Custom Tracking Domain you control over a shared one.

Sources
  1. Web beacon — Wikipedia
  2. Apple advances its privacy leadership with iOS 15, iPadOS 15, macOS Monterey, and watchOS 8 — Apple Newsroom
  3. Turn images on or off in Gmail — Gmail Help
  4. How to set up link branding — Twilio SendGrid Docs
  5. What Mail Privacy Protection Means for Email Marketers — Litmus
External sources open in a new tab.

Related terms

Open Rate

Open rate estimates how many recipients opened an email, but privacy features now inflate it.

Custom Tracking Domain

A custom tracking domain makes tracked links use your own domain instead of a shared one that many other senders also use.

Reply Rate

Reply rate is replies divided by delivered emails, and it is the metric that matters most in cold outreach.

Plain-Text Email

A plain-text email is simple text with no design, which makes cold emails look personal.

Click-Through Rate (CTR)

CTR is the share of recipients who click a link, a common but limited email engagement metric.

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