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.
Hiver - Manage Customer Support, Sales and Projects right from your Gmail inbox. Collaborate with Shared Labels, Shared Contacts, email notes, email reminders and email templates
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.
mailcow - An open source mailserver suite.
MailClark - The Slack bot for external communications