Tracking pixels, explained

Follow the image request

A sender places a remote image reference in an HTML email. When software retrieves that image, the destination server can record a request. A measurement system can associate the request with an identifier in the URL and display a reporting event.

Twilio SendGrid’s tracking documentation describes its one-pixel image implementation and the resulting open event. The event is evidence of retrieval, with limitations that must remain visible in reporting. Read the full tracking pixel walkthrough for the path from message creation to interpretation.

Understand what the identifier represents

A shared image address can produce a count of requests without reliably attributing each one to a delivery. A unique delivery identifier can support more specific grouping, but it still does not verify who requested the image.

Forwarding or copying content can preserve an existing identifier. A later event may therefore remain associated with the original delivery even when the request came through another path. Treat identifiers as links between system records, not authentication. Keep recipient addresses and confidential message content out of image URLs and restrict access to any identifying mapping.

Account for missing and automated requests

A person can read text without loading remote images. Intermediaries can request images on a reader’s behalf, and cached content may be reused without another request to the original host. These possibilities prevent a simple request count from becoming a complete count of human views.

Google’s image handling guidance describes Gmail’s image controls and protections. Verify the specific client and settings involved rather than generalizing from a single test. The absence of an event should remain “not observed,” rather than being relabeled as deliberate inattention.

Decide whether collection is necessary

Write down the decision the measurement would support. Checking whether a resource helps customers may be better served by deliberate feedback or an appropriate completion event. A signature can communicate perfectly well without collecting behavioral records.

If collection is justified in your context, explain it clearly, apply the required permissions, minimize stored fields, and define retention and access controls. Avoid presenting an open event as proof of consent, identity, precise location, or reading. The responsible measurement framework provides practical alternatives and a way to define honest metrics.

Test infrastructure separately from content

A static signature containing an image reference does not create event storage or a reporting service. Those capabilities need separate infrastructure. SigAPI.com explains the concepts and does not operate an active tracking endpoint.

• Use controlled test accounts with documented settings.
• Compare blocked images with explicitly loaded images.
• Repeat a view and inspect the actual origin requests.
• Forward the test to see which identifier remains.
• Disable measurement and confirm the signature still works.

Record observed behavior honestly. A privacy setting preventing collection can be the expected result of the test.