What is pixel tracking for opens?

Pixel tracking (also called beacon tracking) measures email opens by embedding an invisible image that loads from a remote server.

How it works:

1. The email contains a tiny (often 1x1 pixel) transparent image

2. The image URL includes unique identifiers for recipient and campaign

3. When the recipient opens the email and images load, their email client requests the image from the tracking server

4. The server logs the request (timestamp, IP, user agent) and returns the image

5. The sender records an "open" for that recipient

Example tracking pixel:

<img src="https://track.esp.com/open?id=abc123" width="1" height="1">

Limitations:

Only works when images load; many clients block images by default

Proxy caching (Gmail, Apple MPP) can trigger false opens

Multiple opens from preview panes, forwarding, or reopening

Apple Mail Privacy Protection (MPP) fundamentally changed pixel tracking in 2021. Apple preloads all images regardless of whether the user opens, generating false opens for Apple Mail users.

Reliability: Open tracking is now considered an imperfect metric. Use it for trends rather than absolute numbers, and supplement with click data for engagement analysis.