Currently you need to create the 'real' situation in Alma in order to trigger a letter to send. E.g. in the case of making changes to the overdue and lost loan notification letter like sending a message every 2nd day, testing this in the sandbox means you need to setup a test scenario, then make sure you run the correct job every day to see what Alma will be doing. Instead, it would be great if there was a test dialogue that would allow you to specify letter specific relevant parameters, eg. in the above example which user, which items, when the due dates would have been, etc.