Self-Hosted Microsoft 365 Alternative for Businesses: Which Open-Source Building Blocks Replace Files, Office, Chat, Video and Mail

Want to move off Microsoft 365 but not sure what to use instead? WZ-IT's collaboration and Microsoft 365 alternative selection compares the matching open-source building blocks against your requirements. During the selection you try all candidates yourself in demo instances; the result is a decision paper, from €3,900 excl. VAT. Book a free initial consultation

Searching for a Microsoft 365 alternative for a business mostly turns up lists of individual products. Those lists miss the actual question: Microsoft 365 is not one program but a bundle of mail, file storage, office, chat, video meetings and sign-in, all tightly connected. A self-hosted open-source alternative is therefore made of several building blocks that are chosen, connected and operated together.

This guide sorts the building blocks by function, gives the licence and edition limits with figures from vendor sources, shows what can and cannot be migrated from SharePoint, OneDrive, Teams and Exchange, and summarizes the facts on data protection and sovereignty. As of October 2026.

Key points at a glance:

1. No single open-source product replaces Microsoft 365. Functions are replaced, each with a suitable building block.
2. Many open-source products have a free and a paid edition. The limits often concern sign-in, user count or message history.
3. Files and mailboxes can be migrated; Teams chat history usually cannot.
4. The glue is your own identity service with single sign-on.
5. A step-by-step replacement is the norm, not a single cut-over date.

Table of Contents

1. Why no single product replaces Microsoft 365
2. The building blocks at a glance
3. Files and office: Nextcloud or OpenCloud with browser-based office
4. Teams alternative for chat: Matrix, Mattermost, Rocket.Chat or Zulip
5. Video meetings: Jitsi, OpenTalk or Element Call
6. Email and calendar: the building block with the most dependencies
7. Sign-in: Keycloak or authentik as the glue
8. Migrating from SharePoint, OneDrive, Teams and Exchange
9. Ready-made suite or your own combination
10. Operation: what remains after the rollout
11. Data protection and sovereignty: what the sources say
12. Common misconceptions
13. Our approach at WZ-IT
14. Further guides

Why no single product replaces Microsoft 365

Microsoft 365 connects several services through one account: Exchange Online for mail and calendar, SharePoint and OneDrive for files, Word, Excel and PowerPoint in the browser and on the desktop, Teams for chat and meetings, and Entra ID for sign-in and permissions. Its strength lies in this integration, for example when a file from a Teams channel opens in Word and is shared with the same permissions.

Open-source software is organized differently. Each function has specialized projects with their own licence, vendor and edition. Integration is created deliberately: a shared sign-in, browser-based office inside the file platform, video meetings from the chat. Every replacement therefore has to answer three questions:

• Which building block per function? Not every function has to be replaced, and not every building block suits every company size.
• Which edition? The free edition is not always enough, for example for single sign-on, user count or support.
• Who operates the building blocks? Updates, backups and monitoring apply to each building block.

The building blocks at a glance

Prices net or excluding tax, as of October 2026, sources in the following sections. The billing units differ (per user per year, per user per month, per mailbox), so the figures are not directly comparable.

Files and office: Nextcloud or OpenCloud with browser-based office

File storage is the first building block for most companies, because most data lives there and the replacement is the easiest to plan.

Nextcloud is licensed under AGPL-3.0 (GitHub, Nextcloud Server) and is more than file storage: calendar, Talk and office are part of the scope. The current version is 35.0.1 of 24 September 2026, marketed as Nextcloud Hub 26 Summer (Nextcloud Changelog). The community version has no hard user limit. From 1,000 active users without a subscription, the free push service is throttled and a notice is shown at login (Nextcloud, Subscription Registry).

Nextcloud Enterprise comes in three tiers, each from 100 users (Nextcloud Pricing):

Office in Nextcloud is now a choice: Nextcloud Office is based on Euro-Office, and Collabora Online and Thinkfree Office can be connected instead (Nextcloud Office).

OpenCloud is licensed under Apache-2.0 and belongs to the Heinlein Group (OpenCloud, Company). It is written in Go and stores metadata in files rather than a relational database (OpenCloud Features). According to the vendor, the Community Edition can be used in production; subscriptions cost EUR 2, 4 or 8 per user per month as a price example for around 100 users (OpenCloud Subscription). OpenCloud fits when lean file storage is needed and calendar or chat come from other building blocks.

There are three routes to browser-based office:

Sources: Collabora CODE, Collabora Pricing, Nextcloud and Collabora, ONLYOFFICE Changelog, ONLYOFFICE editions, ONLYOFFICE Enterprise prices.

If you work heavily with DOCX, XLSX and PPTX and exchange them with partners, focus on format fidelity and test with real files: templates with fields, spreadsheets with formulas, presentations with master slides. Macros in Excel files are a separate test item. Desktop office is not affected: staff can keep using Word and Excel locally while the files live on the new platform.

The licence limits of Nextcloud, OpenCloud, Seafile and ownCloud are compared in detail in the post Nextcloud, Seafile, OpenCloud or ownCloud.

Teams alternative for chat: Matrix, Mattermost, Rocket.Chat or Zulip

There are four established open-source projects for chat. The biggest differences lie in their architecture and in what the free edition allows for businesses.

Sources: Synapse README, Element Server Suite, Element Pricing, Mattermost editions, Mattermost LICENSE, Rocket.Chat plans, Zulip plans, Zulip Teams import. As of October 2026.

Three points stand out when checking against the vendor information:

• Element Server Suite Community is intended by Element for small, non-commercial installations with up to 100 users and comes without support. For businesses, ESS Pro is intended, with group and lifecycle management via LDAP or SCIM, high availability and support.
• Mattermost has restructured its former free edition. Entry includes the features of the top edition with limits, including 10,000 messages of history across all channels. Team Edition is intended for under 250 users, and single sign-on is being removed from it.
• Rocket.Chat now calls its plans Starter, Commercial, Government and Defense. According to its documentation, the Community edition is not intended for business-critical use.

For a business from around 50 users with company sign-in, a paid edition should therefore usually be planned for Matrix, Mattermost and Rocket.Chat. Zulip is the exception with public prices and the full feature set in the free edition. Business adds unlimited push notifications and support for sign-in via SAML and Entra ID as well as AD and LDAP sync. Mattermost, Rocket.Chat and Zulip are compared directly in the post Slack alternatives: Mattermost, Rocket.Chat and Zulip.

Video meetings: Jitsi, OpenTalk or Element Call

There are three routes for meetings, differing in how closely they are tied to the chat:

Sources: Jitsi Meet LICENSE, OpenTalk on-premises, OpenTalk prices, Element Pricing.

If you choose Matrix for chat, Element Call provides video in the same tool. With Mattermost, Rocket.Chat or Zulip, Jitsi Meet is the usual partner. How Jitsi compares to Zoom is covered in the post Jitsi Meet vs. Zoom.

Email and calendar: the building block with the most dependencies

Mail is the building block whose replacement affects the most parties: Outlook on the desktop, mobile devices, shared mailboxes, calendar sharing, meeting rooms and external deliverability.

Sources: mailcow documentation, Stalwart README, Stalwart changelog, Stalwart Pricing, openDesk README.

The decisive question is Outlook. Email works over IMAP, including in the new Outlook for Windows (Microsoft, feature comparison of new and classic Outlook). Calendar and contacts are harder: according to the mailcow documentation, the new Outlook supports neither CalDAV nor CardDAV, ActiveSync no longer works with mailcow from Outlook 2019 onwards, and classic Outlook needs a plugin such as CalDav Synchronizer (mailcow, Outlook). Businesses that want to keep Outlook including calendars clarify this before choosing a mail server. Alternatives are webmail with an integrated calendar, or Thunderbird.

Which of the three self-hosted mail servers suits which business is compared in the post Mailcow, Stalwart or Mailu. Many businesses replace mail last and keep Exchange Online during the transition, while files, chat and video have already moved.

Sign-in: Keycloak or authentik as the glue

What Entra ID does in Microsoft 365 is handled by your own identity service in an open-source environment. It is the glue that turns separate building blocks into one workplace: one account, one sign-in with multi-factor authentication, groups and permissions in one place, leavers handled in one place.

Sources: Keycloak, CNCF, Keycloak, Keycloak 26.8.0, authentik LICENSE, authentik Pricing.

During the transition, the identity service can use Entra ID as its sign-in source. The new building blocks then authenticate against the identity service, and staff keep using their existing accounts. Once Microsoft 365 is replaced, the identity service manages the accounts itself or connects the on-premises Active Directory. The two tools are compared in the post Authentik vs. Zitadel, and the Keycloak upgrade path in Keycloak 26.8 upgrade guide.

Migrating from SharePoint, OneDrive, Teams and Exchange

What can be migrated depends on the service:

Some points in detail:

• Files: rclone supports OneDrive Personal, OneDrive for Business and SharePoint document libraries. rclone can read and write permissions only between OneDrive or SharePoint remotes; for Nextcloud or OpenCloud, sharing is set up again (rclone, OneDrive). Nextcloud offers migration from SharePoint, OneDrive, Teams and Exchange as a supported project, preserving permissions and metadata "depending on the platform" (Nextcloud, Migrate). Nextcloud does not offer a freely available tool for this.
• Mailboxes: Exchange Online has disabled Basic Authentication for IMAP, POP, EWS and ActiveSync in all tenants (Microsoft Learn, Basic Authentication). A migration therefore requires OAuth2. imapsync supports this with a pre-generated access token.
• Teams: Microsoft provides export APIs for one-to-one, group and meeting chats as well as channel messages, and allows the export at no charge when switching to another provider. For tenants with up to 500 users there is also an export tool (Microsoft Learn, Export Teams content, Microsoft Learn, Migration from Teams). Mattermost's mmetl imports only from Slack and Rocket.Chat (GitHub, mmetl), Rocket.Chat only from Slack and CSV (Rocket.Chat, Data Migration). The chat history is therefore usually kept as an archive, and channels start fresh in the new chat.

Order: It has proven useful to set up the identity service first, because all further building blocks depend on it. File storage and office follow, then chat and video, and mail and calendar last. Each step starts with a pilot group, and Microsoft services keep running in parallel during the transition.

Ready-made suite or your own combination

Besides your own combination there are ready-made suites. The best known is openDesk by ZenDiS with Element, Nextcloud, OX App Suite, Collabora, Jitsi, OpenProject, XWiki, CryptPad, Notes and Nubus as portal and identity management (openDesk README). There is a Community and an Enterprise Edition; for production use the vendor recommends the Enterprise Edition with additional modules and support (openDesk Enterprise Edition).

What the openDesk partner programme means for small and medium-sized businesses is covered in the post openDesk partner programme.

The state administration of Schleswig-Holstein shows what a full switch looks like at scale: on 2 October 2025 the move from Microsoft Exchange and Outlook to Open-Xchange and Thunderbird was completed, covering more than 40,000 mailboxes and more than 100 million emails and calendar entries, after six months. Nextcloud is gradually replacing SharePoint there, and video meetings run on OpenTalk (Schleswig-Holstein, milestone for digital sovereignty, in German).

Operation: what remains after the rollout

Microsoft 365 is consumed as a service; updates and availability are the vendor's responsibility. In an open-source environment these tasks lie with your own team or your operator:

The more building blocks, the more updates and dependencies. A selection of a few well-integrated building blocks is therefore usually cheaper to operate than a broad collection. The environment runs on your own infrastructure or with an operator in Germany, for example via Managed Open Source.

Data protection and sovereignty: what the sources say

The debate about Microsoft 365 is often conducted in slogans. The facts, as of October 2026:

Two factual conclusions follow. First, storage location in the EU alone does not answer who can compel access; under the CLOUD Act, what matters is the provider's control over the data. Second, data transfers to the US currently have a legal basis in the Data Privacy Framework, whose continued existence depends on future decisions. With self-hosted open-source software, the data is at the disposal of your own company or your chosen operator. The question of access then depends on that operator's seat and jurisdiction.

For many businesses the driver is less the legal situation than dependence on a single vendor for pricing, licence terms and features. The background is described in the post Move servers and data to European clouds.

Not legal advice. For specific questions, consult data protection or legal counsel. Trademarks belong to their respective owners.

Common misconceptions

Our approach at WZ-IT

WZ-IT supports the replacement from decision to operation. Every step has a defined result.

1. Orientation when the topic is still open. If it is not yet clear whether and in which order Microsoft 365 will be replaced, the orientation workshop sorts the topics. It takes place remotely as a half-day session and costs from EUR 1,490 net.
2. Selection with a defined scope. The collaboration and Microsoft 365 alternative selection costs from EUR 3,900 net. It covers an assessment of your Microsoft 365 usage, weighted criteria with business units and IT, and a review of licence and edition limits. For the duration of the selection we provide demo instances of all candidates, which you try out yourself without committing first. The result is a decision paper with a recommendation per building block, licence costs from vendor information, migration path and operating model. If a different solution fits better, the paper says so. The amount is credited towards the rollout if you commission it within 6 months on the same topic.
3. Rollout and migration. You receive the quote for this together with the decision paper. It covers the identity service and single sign-on, the chosen building blocks, the migration of files and mailboxes, pilot groups and the switch building block by building block. Licences for paid editions are purchased directly from the vendor.
4. Operation in Germany. We operate the building blocks via Managed Open Source in German data centres, from EUR 129.90 net per workload per month, with updates, backups and monitoring. Alternatively they run on your own infrastructure.

All entry points from the free initial consultation to the selection are listed on the page Consulting at WZ-IT.

Further guides

• Nextcloud, Seafile, OpenCloud or ownCloud, licence limits and ownership of file platforms.
• Slack alternatives: Mattermost, Rocket.Chat and Zulip, the team chats compared.
• Mailcow, Stalwart or Mailu, running your own mail server.
• Authentik vs. Zitadel, identity services for single sign-on.
• openDesk partner programme, what the German federal suite means for SMEs.
• Introducing a company wiki, when SharePoint pages are replaced as the intranet.
• Collaboration and Microsoft 365 alternative selection, WZ-IT's package.

Replace Microsoft 365 without switching everything at once. We clarify with you which services to replace, let you try the matching building blocks yourself in demo instances, and support migration and operation. Book a free initial consultation

Sources

• GitHub, Nextcloud Server
• Nextcloud Changelog
• Nextcloud Pricing
• GitHub, Nextcloud Subscription Registry
• Nextcloud Office
• Nextcloud and Collabora
• Nextcloud, migration from Microsoft 365
• OpenCloud, Company
• OpenCloud Features
• OpenCloud Subscription and Support
• Collabora Online Development Edition
• Collabora Online Pricing
• GitHub, ONLYOFFICE DocumentServer Changelog
• ONLYOFFICE, compare editions
• ONLYOFFICE Docs Enterprise, prices
• GitHub, Synapse README
• GitHub, Element Server Suite
• Element Pricing
• Mattermost, Editions and Offerings
• GitHub, Mattermost LICENSE
• GitHub, mmetl
• Rocket.Chat, Our Plans
• Rocket.Chat, Data Migration
• Zulip Plans
• Zulip, Import from Microsoft Teams
• GitHub, Jitsi Meet LICENSE
• OpenTalk on-premises (German)
• OpenTalk prices (German)
• mailcow documentation
• mailcow, Outlook
• GitHub, Stalwart README
• GitHub, Stalwart Changelog
• Stalwart Pricing
• Microsoft Support, feature comparison between new Outlook and classic Outlook
• Keycloak
• CNCF, Keycloak
• GitHub, Keycloak 26.8.0
• GitHub, authentik LICENSE
• authentik Pricing
• rclone, Microsoft OneDrive
• imapsync, FAQ Office 365
• Microsoft Learn, Deprecation of Basic authentication in Exchange Online
• Microsoft Learn, Export content with the Microsoft Teams Export APIs
• Microsoft Learn, Migration from Teams
• openDesk README
• openDesk Enterprise Edition
• 18 U.S.C. § 2713, Legal Information Institute
• Microsoft On the Issues, EU Data Boundary completed
• Microsoft Learn, What is the EU Data Boundary
• European Commission, IP/23/3721
• General Court, press release 106/25
• Schleswig-Holstein, milestone for digital sovereignty (German)