When you send an email with open tracking / read receipts enabled, Mailspring embeds a tiny 1px image in the body of the email. Each time a recipient opens the email, their browser or mail client loads the image and Mailspring records that it's been viewed.
Some mail clients cache the 1px image, meaning you'll only see one "open" the first time they viewed the email. Most don't though—each time the recipient reads the email, you'll get another notification.
Mailspring automatically silences open and link tracking notifications if they're within a few minutes of the previous one to avoid flooding you with notifications if your recipient has a habit of flipping back and forth between emails. In the future, we may add more settings so you can silence repeated notifications entirely. Sometimes it's fun to know that your recipient read your email three times, sometimes it's annoying.