iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Spark Mail - Spark helps you take your inbox under control. Instantly see what’s important and quickly clean up the rest. Spark for Teams allows you to create, discuss, and share email with your colleagues
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.
Polymail - Native email app for email productivity.
mailcow - An open source mailserver suite.
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.