A hidden pixel is placed within the HTML of your email, which enables us to track the number of opens.
Once the images are downloaded within the email, the open is registered.
If a recipient clicks a link (even without downloading the images), technically the subscriber has seen your content and has opened it to click, but we won’t track this unless the image has been downloaded, we will track the click as normal.
If your email isn’t HTML, but plain text, then it won’t include the image pixel and unfortunately, we cannot track opens. We can track all replies and clicks as usual though.