Bots UI Kit - Fully customizable Sketch UI Kit for Messenger Platform
Templates by Email Monster - Hundreds of free, responsive email templates. Pick and edit.
Now UI Kit - A beautiful Bootstrap 4 UI kit. Yours free.
MJML - The open source framework for responsive emails
lstore.graphic - Mockup Scene Creator
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.