MJML - The open source framework for responsive emails
Paperplane - High quality HTML to PDF conversion API for developers.
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.
Foundation for Emails 2 - Create responsive HTML emails that work on any device
Unlayer - Embeddable email editor and page builder for SaaS and CRMs
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.