The best open source alternative to Zimbra Collaboration Suite is Roundcube. If that doesn't suit you, our users have ranked more than 25 alternatives to Zimbra Collaboration Suite and many of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Zimbra Collaboration Suite are mailcow: dockerized, Nextcloud Hub, Kolab and Horde.
Roundcube webmail is a browser-based multilingual IMAP client with an application-like user interface. It provides full functionality you expect from an e-mail client, including MIME support, address book, folder manipulation, message searching and spell checking.
mailcow is a mail server suite based on Dovecot, Postfix, SOGo, Rspamd and other open source software, that provides a modern Web UI for administration.
Nextcloud is the most deployed on-premises file share and collaboration platform. Access & collaborate across your devices. As fully on-premises solution, Nextcloud Hub provides the benefits of online collaboration without the compliance and security risks.
Enterprise-level, browser-based suite for IMAP email, calendar, contacts, tasks, notes, wiki, and file management, offering syncing, shared and unlimited resources, multilingual support, customizable dashboard, and integration with mobile and external clients.
The OX App Suite by Open-Xchange is web-based platform offering a suite of apps like an email client, calendar, contacts magement, cloud storage with synchronization and productivity apps for word processing, spreadsheets and presentations.
Kolab Now provides a suite of intuitive apps in a user friendly environment to make your day to day online comms easier, smarter and most importantly; secure.
Carbonio Community Edition (Carbonio CE) is a free and open-source (FOSS) email and collaboration platform designed by Zextras. It is a complete email solution, allowing organizations to maintain digital sovereignty through on-premises self-hosted servers, ensuring privacy and...
grommunio (formerly known as grammm) efficiently combines all requirements of modern, digital communication and collaboration.
Cloudflare Workers-based email solution providing multiple address generation, attachment storage via R2, responsive web interface, RBAC management, Resend bulk email, Telegram or third-party forwarding, verification code recognition, and ECharts analytics.
Blue Mind is a complete solution of company messaging, calendars and collaborative work. It aims to enable you to work daily with comprehensive, useful, efficient and effective tools.
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Roudcube is only web mail client not groupware solution like Zimbra