MJML is a markup language to build responsive email templates by providing a semantic syntax and a rich standard component library.
Install the package:
composer require punktde/neos-mjml
Install / configure the MJML compiler
You can either install the mjml binary globally and configure its path in the Settings.yaml or install the binary locally with npm:
npm install --prefix Packages/Application/PunktDe.Neos.Mjml/Resources/Private/Library