Substack detects opens by watching users load the images in their messages or opening a post after receiving a mobile app notification about it. Often this happens when the user clicks an email, but sometimes images can load in previews, or even be eagerly loaded by email clients for messages at the top of a user's mailbox. In addition, users can change their mailer settings to decline to load images or read text-only versions of the messages that don't load images at all.
Each email sent by Substack is marked with its intended recipient. We track opens by the intended recipient, not by the email messages themselves, so a reader who forwards their email to a friend will appear as a single user, opening a message twice.
The "open rate" percentage is the percentage of people who opened that particular post after receiving an email or Substack app notification about it. If one person opens your post five times, that counts as one open.