Zimbra Collaboration
Email, calendar, collaboration server with Web interface and lots of integrations.
Alternatives
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Openmeetings
Video conferencing, instant messaging, whiteboard, collaborative document editing and other groupware tools using API functions of the Red5 Streaming Server for Remoting and Streaming.
egroupware
Software suite including calendars, address books, notepad, project management tools, client relationship management tools (CRM), knowledge management tools, a wiki and a CMS.
Citadel
Groupware including email, calendar/scheduling, address books, forums, mailing lists, IM, wiki and blog engines, RSS aggregation and more.
Colanode
Open-source and local-first Slack and Notion alternative that puts you in control of your data
Cozy Cloud
Personal cloud where you can manage and sync your contact, files and calendars, and manage your budget with an app store full of community contributions.
DavMail
POP/IMAP/SMTP/Caldav/Carddav/LDAP exchange gateway allowing users to use any mail/calendar client with an Exchange server, even from the internet or behind a firewall through Outlook Web Access.