Froala Design Blocks - 170+ free responsive design blocks based on Bootstrap 4
HTMLEmail.io - Responsive HTML email templates for startups & developers
Webflow Flexbox UI Builder - Build flexible, responsive layouts — without writing code
Templates by Email Monster - Hundreds of free, responsive email templates. Pick and edit.
Antetype - Antetype - Layout aware User Interface Design - responsive and future proof.
MJML - The open source framework for responsive emails