MJML - The open source framework for responsive emails
Paperplane - High quality HTML to PDF conversion API for developers.
BeeFree.io - Bee is MailUp's drag-&-drop editor for responsive design emails.
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.
BeeFree - BeeFree is an all-in-one solution that works anywhere you are.
Foundation for Emails 2 - Create responsive HTML emails that work on any device