MJML - The open source framework for responsive emails
Mockups Design - Mockups Design is the most leading web-based application that comes with a vast collection of creatively design mockups for all kinds of products and devices.
Stripo - Stripo is an all-in-one email design platform. We enable our clients to build emails of any complexity, including interactive AMP emails, really fast and easy.
Mr. Mockup - Mr.
Templates by Email Monster - Hundreds of free, responsive email templates. Pick and edit.
Pixeden - Pixeden is an all-in-one premium design and web resources platform that provides high-quality graphic design templates and lots of other files.