How do email read-receipts work?

I am trying to implement my own simple read-receipts feature for gmail.

How does it work?

component 1: Tracking pixel embedded in the email

component 2: Free deno deploy serverless endpoint to keep the count

My first goal is to demonstrate that opening the email does trigger the endpoint, registering the read count.

Reference img tag I embed in the email (by appending this element as a child to the gmail text editor div element) <img style="display: none; height: 1px; width: 1px;" src="https://my-custom-deno-deploy-endpoint.deno.dev" />

The problem

Gmail is caching the img src (my deno endpoint) thus breaking my logic.

Question:

Is there a workaround? How do the established apps do this?