Rebound for Postmark is a new integration that allows customers to be notified about email delivery issues from their web app, providing a more seamless experience. To get started, users need to sign up for a Postmark account and follow a step-by-step process to set up Rebound, which involves authorizing access to the Postmark account, choosing a server, editing notification appearance and messaging, and installing a custom JavaScript snippet near the closing `</body>` tag of their web app. The integration has some limitations, such as only triggering notifications for hard bounces and not spam complaints, but also offers flexibility through its callback function API. Users can provide feedback on Rebound to help improve future updates.