For Insights, the Open metric is based on the downloading of images. This means when the images inside the Blast are downloaded in the Subscriber's inbox, an open WILL be tracked for that specific Subscriber.
Two important things to note about Subscribers registering opens:
If the images do not download when a Subscriber is viewing their email, they will need to download the images in their inbox in order to register an open.
If Subscribers are viewing the email in it's preview pane, if they can see the images, they WILL register an open.
To learn more about the difference between an Open and a Unique Open, check out this article.
Please Note: Apple released a privacy settings update. To see how this may impact your Insights, check out this article.
If you have any questions at all, please don't hesitate to reach out to us at support@cerkl.com or use the support Chat toward the bottom right-hand corner of any cerkl.com page.