We're evaluating whether to replace Microsoft 365 with a fully self-hosted collaboration and groupware environment for five companies. The main drivers are data sovereignty and reducing dependence on Microsoft. The environment would include roughly 100–160 users and mailboxes across five domains, about 2 TB of mail and file data, on-premises hosting in redundant data centers, centralized authentication, and high-availability and backup requirements. Keeping Outlook is not necessary. Ideally, the solution would provide email, webmail, calendars, contacts, chat, audio and video conferencing, persistent voice/video rooms, file sharing, collaborative Office-document editing, and clear separation between the companies with shared cross-company spaces. Products under consideration include Nextcloud, ownCloud or OpenCloud, Matrix, Mattermost, Open-Xchange, Kopano, Collabora, and ONLYOFFICE. I'm especially interested in real-world deployments: what stack are you using, what works well, what compromises did you make compared with Microsoft 365, and what do you wish you had known before starting?
5 Answers
The main warning is not that this is impossible; it’s that Microsoft 365 is an integrated suite built from many mature services. Replacing it with one self-hosted product usually means assembling separate systems for identity, mail, calendars, files, chat, conferencing, document editing, monitoring, backups, and mobile notifications. The integration and operational burden become your responsibility. Before choosing products, split the project into identity, email and calendaring, files and collaboration, messaging, and conferencing. Define which features users actually depend on and which can be dropped. A narrowly scoped system is much more realistic than trying to reproduce every part of Microsoft 365.
Do not underestimate the migration itself. Moving mail through IMAP is manageable, and files can usually be copied, but shared links, permissions, embedded references, chat history, and files attached to conversations often do not transfer cleanly. Decide early what must be migrated, what can become a read-only archive, and how long the old systems will remain available. The two-terabyte data move should be treated as a separate project with its own testing, validation, rollback, and user-acceptance plans.
There are examples of organizations using combinations such as Dovecot for mail, Seafile for storage, Collabora for editing, Matrix for messaging, and a dedicated conferencing system. OpenDesk-style bundles and products such as IceWarp or Zimbra can also cover more of the groupware surface in one package. They can be workable, but none will provide the same breadth, polish, ecosystem, and low-friction integration as Microsoft 365. The strongest business case is a specific sovereignty, legal, or air-gap requirement—not simply replacing Microsoft out of frustration. Compare the yearly operating cost and staffing requirement against the cost of a hosted alternative before committing.
Email is likely to be the biggest operational risk. Deliverability, IP reputation, spam filtering, malware scanning, DKIM, SPF, DMARC, backups, restores, and account recovery all need constant attention. Several people recommend keeping an external mail relay or security service even when the rest of the environment stays on-premises. Also budget for at least two experienced engineers, monitoring and automation, disaster recovery, documentation, patching, and user support. The hardware may already be available, but people and long-term operations are the expensive part.
We’re trying to move away from Microsoft products broadly, not only from cloud hosting. Windows would remain for VDI and some terminal servers, but most server workloads would run on Linux.
A plausible modular stack could use Keycloak or Authentik for identity, Dovecot with a suitable SMTP service for mail, Seafile or Nextcloud for files, Collabora or ONLYOFFICE for document editing, and Mattermost or Matrix for messaging. BigBlueButton or another dedicated conferencing platform could cover video meetings. Open-Xchange, Zimbra, IceWarp, or similar commercial groupware products may reduce the amount of integration work on the mail and calendar side. Mattermost is closer to a Slack-style workspace, while Matrix is more flexible for persistent rooms and federation, but neither will reproduce the entire Microsoft experience without trade-offs.
For a fully self-hosted deployment, pay close attention to mobile push notifications, especially for chat. Some platforms rely on Apple and Google push infrastructure, so the self-hosting requirement may limit the mobile experience.
A phased rollout would help. Start with one company, measure support requests and administrative effort for a few months, then decide whether the remaining companies should follow.