What are some alternatives?
When comparing Responsive Email and Am I Responsive, you can also consider the following products
Adapti - Create a unique web experience for each of your users
W3C Mobile Checker - Make websites work better on mobile devices
Responsive View - Test your responsive design across different devices
LT Browser by Lambdatest - LT Browser allows you to perform live mobile view debugging of your website on different devices. You can test on mobile and tablet simultaneously on our browser and debug user issues and see their impact on the go on a wise range of mobile devices.
SlideRule UX Design Path - Curriculum of Online Courses To Learn UX Design
Responsive Design Checker - Check how responsive is your website across device sizes