Mail Designer 365 - Discover Mail Designer 365, the easiest way to design mobile-optimized HTML emails, templates and newsletter designs. Your HTML email designer for Mac.
MJML - The open source framework for responsive emails
Really Good Emails - A large collection of good product email design.
Email Design System for Sketch - Sketch file with 10 email templates for designers
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HTMLEmail.io - Responsive HTML email templates for startups & developers