How do Boomerang for Android's Read Receipts work?

When you use Boomerang to request a Read Receipt on an email, it will add a read receipt notification to the bottom of the message, and will also track clicks on any links you include in the message. Shortly after the message is read, you’ll receive a notification email from us that the recipient has read your message.

Unlike the open tracking approach of inserting an invisible 1x1 tracking pixel, Boomerang embeds an image that is fully visible in the email, explaining the sender requested a read receipt and that the recipient can opt out by clicking on the link in the image. That way, the default read receipt behavior is to provide it, but the recipient still has the option to opt out.