When2Meet - When2meet helps you find the best time for a group to get together. It is a free survey tool that is quick and easy to use.
Zimbra - Zimbra is trusted by over 500 million users to increase productivity with a complete set of collaboration tools while maintaining total control over security and privacy.
x.ai - x.ai is a tool to schedule meetings for you and your team.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
WhenAvailable - WhenAvailable is an excellent event management tool that allows scheduling the dinner party, book club meeting, and guys’ night out with simple and best ideas.
mailcow - An open source mailserver suite.