Salesforce provides a great functionality that allows you to create a sandbox of your production Salesforce organization with the click of a few buttons. When you do that, you might want to test out Soapbox Mailer in that newly created sandbox. Your Soapbox Mailer app will work in a sandbox, but there are a few things to keep in mind when you're doing this.
1) Sender records would need to be created in the sandbox to ensure there are senders.
2) Email Template records would need to be created in the sandbox.
3) The Amazon Settings would need to be populated again in the sandbox
4) The Email Services setup would need to be created in the sandbox, and unique Amazon SNS topics would need to be created for the new email service address provided by Salesforce
With those changes, you should be able to use Soapbox Mailer in your sandbox account.