BeeFree.io - Bee is MailUp's drag-&-drop editor for responsive design emails.
mailcow - An open source mailserver suite.
Topol.io - Create professional Email templates easily - Drag & Drop Email Builder.
hMail Server - hMailServer is a free e-mail server for Microsoft Windows.
MJML - The open source framework for responsive emails
SmarterMail - SmarterMail is a next generation email management server that is offering several business collaboration and communication features as well.