Substack detects opens by watching users load the images in their messages. 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" we show in the interface is the proportion of intended recipients who have opened the email. Below the open rate we show the absolute number of opens, but an individual opening the same email many times (or sending it to their friends) may count for multiple entries in that absolute number.