What are some alternatives?
When comparing Responsive Email and SmashBack, you can also consider the following products
Adapti - Create a unique web experience for each of your users
MJML - The open source framework for responsive emails
Responsive View - Test your responsive design across different devices
Foundation for Emails 2 - Create responsive HTML emails that work on any device
SlideRule UX Design Path - Curriculum of Online Courses To Learn UX Design
Stripo - Stripo is an online drag-n-drop and HTML email editor to design responsive email templates fast without any HTML skills.