Tabler Email UI Kit - Responsive newsletters and email templates 💌
HTMLEmail.io - Responsive HTML email templates for startups & developers
PutsMail - Test your HTML emails before sending them
Templates by Email Monster - Hundreds of free, responsive email templates. Pick and edit.
MailChimp - MailChimp is the best way to design, send, and share email newsletters.
MJML - The open source framework for responsive emails