What Is an Email Tracking Pixel?
A tracking pixel is typically a 1x1 invisible transparent GIF or PNG image hosted on an external server and referenced in email HTML:
<img src="https://tracker.example.com/pixel.gif?user_id=12345" width="1" height="1" />
When your email client renders this image, it sends an HTTP request to the tracker’s web server, transmitting: - Your exact public IP address and approximate geographic location. - The exact timestamp when you opened the email. - How many times you re-opened or forwarded the email. - Your device type, OS, and browser user-agent.
The Difference Between Inline CID Images and Remote Images
Not all images in an email are privacy risks:
- Embedded CID Images (cid:image001.png): These images are stored directly inside the archive file as MIME attachments. Rendering them requires zero network requests and causes zero privacy leaks.
- Remote Images (https://...): These images reside on external web servers. Fetching them contacts third-party domains over the public internet.
How Online Email Viewer Shields Your Privacy
By default, our email viewer blocks all external HTTP/HTTPS image requests, escaped CSS background URLs, and relative paths via strict HTML sanitization and an isolated iframe sandbox with Content-Security-Policy: default-src 'none'; style-src 'unsafe-inline'; img-src data: blob:;. Embedded CID images resolve purely in browser memory as local blob URLs. If you deliberately trust the sender, you can explicitly load remote images per-message.