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