MJML - The open source framework for responsive emails
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Topol.io - Create professional Email templates easily - Drag & Drop Email Builder.
BeeFree.io - Bee is MailUp's drag-&-drop editor for responsive design emails.
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.
HTMLEmail.io - Responsive HTML email templates for startups & developers