When sending an email you receive a message opened webhook immediately, additionally you do not receive any further message.opened notifications from that recipient.
We have observed the following behavior:
- CDNs cache the image pixel across multiple proxies resulting in multiple reads.
- Gmail caches the images the moment they receive them, returning in message.opened webhook.
- Opening one email can trigger all the other emails stored by that provider to also trigger
- This can also result in multiple message.opened, message.link_clicked webhooks being recorded
To get around this it is advised that you disregard immediate responses and store the objects in a SET to eliminate duplicates. Please note that tracking is becoming harder as browsers and providers harden privacy.