
MJML leverages high level components to abstract the complexity of responsive email. It embeds all of the hacks needed to make emails responsive everywhere, from Outlook 2003 to Inbox by Gmail: conditional comments, media queries and fallbacks. Based on a semantic syntax, it’s very easy to learn, use and maintain.
It used to take me days to create new emails, and there aren't very many email builders out there that will allow a lot of customization without having in-depth knowledge of coding/HTML. MJML is a god-send and it is completely free and open-sourced.