mailcow - An open source mailserver suite.
Templates by Email Monster - Hundreds of free, responsive email templates. Pick and edit.
hMail Server - hMailServer is a free e-mail server for Microsoft Windows.
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.
SmarterMail - SmarterMail is a next generation email management server that is offering several business collaboration and communication features as well.
MJML - The open source framework for responsive emails