Sandbox Mode has been introduced in Postmark, allowing users to test and experiment with the service without risking accidental sends to real customers or damaging sender reputations. Sandbox Servers are a safe testing environment for emails, delivering all test messages to a black hole and not sending actual email to recipients. Users can set up a new testing environment by selecting the "Sandbox" option when creating a new Server. This feature also includes bounce testing, allowing users to generate fake bounces and test Postmark's handling of different types of bounces. The team is open to feedback and considering additional testing features, including ways to test events like opens, clicks, or unsubscribes.