What are some alternatives?
When comparing Responsive Email and Automated Responsive Design Testing, you can also consider the following products
Adapti - Create a unique web experience for each of your users
Responsive View - Test your responsive design across different devices
SlideRule UX Design Path - Curriculum of Online Courses To Learn UX Design
Rapid Responsive Design - Kill your artboards, design for all sizes faster
Am I Responsive - see your website in multiple screen resolutions
Auto-Layout for Sketch - Responsive design for Sketch