Resources · Technology
Technology stack
and licensing
The open-source projects Office Unbound is evaluating, what each would do in the platform, and the license each one is published under, including the commercial editions, network obligations and trademark rules that matter to anyone building on or hosting them.
Project status Every component below is a candidate under evaluation, apart from the Proxmox VE deployment target. None is integrated into Office Unbound today. The license review is part of Phase 1 and is ongoing.
Build on, don’t rebuild
Most of the platform already exists as mature open-source software. The work is joining it up.
Mail transport, Exchange-compatible groupware, collaborative document editing, file synchronization and federated messaging are each hard problems, and each has an open-source project with years of production use behind it. Office Unbound does not intend to write a new mail server or office suite. Its own code is the part those projects do not provide when used separately: one interface across them, one identity, provisioning and administration that span every service and every tenant, and migration tooling from Microsoft 365 and Google Workspace.
That choice has a cost. The platform inherits each component’s release schedule, security advisories, architecture and license. So selection is about more than features. A component has to behave well with many tenants, expose the APIs the administration layer needs, and come under a license that allows the way Office Unbound and the MSPs hosting it intend to use it. The second half of this page is about that last point.
How the pieces fit
The custom Office Unbound layer sits over candidate components grouped by function, on Linux virtual machines on Proxmox VE.
Components under evaluation
The full candidate list by function. Where two candidates share a row group, they are alternatives or complements, and evaluation will decide which.
- Available Available and verified in a release
- In development Being built now
- Planned In the roadmap, not yet started
- Under evaluation Being assessed; not yet selected
| Function | Component and what it would do | Status |
|---|---|---|
| Virtualization | Proxmox VE: hypervisor and cluster manager for the virtual machines each service runs in, with storage, networking, backup and high-availability features of its own. | Initial target |
| Operating system | Linux: the guest operating system for every service VM. The specific distribution is part of the evaluation. | Initial target |
| Identity and SSO | Keycloak: single sign-on over OpenID Connect and SAML, MFA, user federation and per-tenant realms in front of every service. | Under evaluation |
| Directory | Samba AD / LDAP: a directory for users and groups, and Active Directory interoperability where an organization needs it. | Under evaluation |
| Exchange-style mail | grommunio / Gromox: a groupware server implementing MAPI over HTTP, EWS and ActiveSync alongside IMAP, POP3, CalDAV and CardDAV, which makes it the candidate for Outlook desktop compatibility. | Under evaluation |
| Alternative mail | Stalwart: a mail and collaboration server written in Rust, with JMAP, IMAP, SMTP, CalDAV and CardDAV and built-in spam filtering. | Under evaluation |
| Mail transport | Postfix: a long-established mail transfer agent for routing and relay where the mail server does not handle transport itself. | Under evaluation |
| Spam filtering | Rspamd: spam filtering with SPF, DKIM, DMARC and ARC checks, reputation and statistical classification. | Under evaluation |
| Malware scanning | ClamAV: antivirus scanning of attachments in transit. | Under evaluation |
| File collaboration | Nextcloud: personal and shared storage, sharing, versioning and the integration point for document editors. The leading files candidate. | Under evaluation |
| Desktop sync | Nextcloud Desktop and other suitable open-source client technology: the basis for a branded Office Unbound sync client on Windows, macOS and Linux. | Under evaluation |
| Office editing | Collabora Online: browser-based collaborative editing built on LibreOffice technology. | Under evaluation |
| Office engine | LibreOffice: the office suite behind Collabora Online, and a desktop editor in its own right, with document conversion. | Under evaluation |
| Office editing | ONLYOFFICE Docs: an alternative browser-based editor built around the Office Open XML formats. | Under evaluation |
| Team messaging | Matrix with the Synapse homeserver: an open, federated standard for chat, rooms and channels. | Under evaluation |
| Messaging clients | Element: web, desktop and mobile Matrix clients, as a basis for the chat experience. | Under evaluation |
| Real-time media | MatrixRTC: the Matrix proposal (MSC4143) for calls and meetings signalled over Matrix rooms. | Under evaluation |
| Media server | LiveKit: a WebRTC media server (SFU) for video meetings and screen sharing, and the media backend used by Matrix’s Element Call. | Under evaluation |
| Telephony | FreeSWITCH and SIP: a softswitch for extensions, call routing, queues and voicemail, connecting to SIP trunk providers. | Under evaluation |
| Provisioning and administration | Office Unbound services: the custom layer for tenant provisioning, the management API, policy, the unified interface and migration. This is the code Office Unbound itself will publish. | Planned |
| Enhanced mail security | MailThreatZero: an optional additional layer of phishing and threat filtering, as a separate service. | Under evaluation |
| MSP integration | clientst0r API integration: provisioning and status APIs that MSP platforms such as clientst0r can call. | Under evaluation |
“Under evaluation” means not selected. A component can be rejected on licensing, multi-tenant behavior, API coverage or operational cost, and decisions will be recorded in project updates.
Licensing review
The license of each candidate, checked against its official repository or website in November 2026, with the points that matter for a platform that will be redistributed, rebranded and hosted for others. Open source does not mean “free of conditions”. Several of these projects pair an open-source edition with a commercial one, and several restrict the use of their names.
| Component | License | Commercial, branding and redistribution notes |
|---|---|---|
| grommunio / Gromox | AGPL-3.0; grommunio-web AGPL-3.0-or-later | Network copyleft. Gromox includes Samba-derived code under GPL-3.0. grommunio GmbH sells subscriptions with long-term maintenance and support, and states that “grommunio” and “gromox” are its trademarks. We found no published trademark policy for third-party use; that is a question to settle before any grommunio-based service carries other branding. Repository · Legal notice |
| Stalwart | AGPL-3.0-only or the proprietary Stalwart Enterprise License | Open-core. A set of enterprise features, including multi-tenancy and SCIM provisioning, is available only under the commercial license. For an MSP-oriented platform that is a material point: the open-source edition alone may not cover multi-tenant hosting. The project states that existing open-source features will not move to the enterprise edition. Repository · Enterprise |
| Collabora Online / CODE | MPL-2.0 | File-level copyleft: changes to MPL files must be shared, but the code can be combined with other software. CODE, the Collabora Online Development Edition, is described by Collabora as suited to testing, home use or small teams and “not recommended for production environments”. Supported production use is the commercial Collabora Online subscription. Branding terms for CODE are still to be reviewed. Repository (mirror) · About CODE |
| LibreOffice | MPL-2.0 | Distributed under MPL-2.0. Contributions are licensed under both MPL-2.0 and LGPL-3.0-or-later, and the suite bundles third-party parts under various other licenses, which a redistributor has to carry through. Licenses |
| ONLYOFFICE Docs | Community Edition AGPL-3.0-only with additional terms; Enterprise and Developer editions commercial | The additional terms (under AGPL section 7) require keeping notices and attribution, marking modified versions as modified and based on ONLYOFFICE, and showing notices in the interface that identify ONLYOFFICE as the original developer. They grant no rights to ONLYOFFICE trademarks or logos. Own-brand (“white label”) use is a commercial Developer edition feature. The Community Edition comparison lists mobile web editors, the admin panel, clustering and multi-tenancy as not included. In May 2026 the project removed the long-standing 20-connection limit from the Community source, though its documentation still recommends the edition for up to 20 users. We have not confirmed which release first ships that change. Repository · Trademark policy |
| Nextcloud server | AGPL-3.0-or-later | Network copyleft. Nextcloud’s trademark rules treat custom theming or removing default functionality as a modified version that may not use the Nextcloud marks. Advertising hosted Nextcloud VM or container images also needs permission. A platform that presents Nextcloud under its own interface must follow those rules. Repository · Trademarks |
| Nextcloud desktop client | GPL-2.0-or-later | A redistributed build, modified or not, may not put “Nextcloud” in its name and must state that it is not the official client. A branded Office Unbound client is therefore consistent with the rules, provided it is clearly not presented as Nextcloud’s and the GPL source obligations are met. Mobile apps redistributed through app stores have similar rules. Repository |
| Matrix Synapse | AGPL-3.0-or-later or Element commercial license | Synapse was previously published under Apache-2.0. In late 2023 Element moved its development to element-hq/synapse under AGPL-3.0, with a contributor license agreement that lets Element sell commercial licenses as an alternative. Current development is AGPL. Operators who modify it and serve users must offer the modified source. Repository · Element announcement |
| Element clients | Element Web: AGPL-3.0-or-later or GPL-3.0-or-later or commercial; Element X (Android, iOS): AGPL-3.0-or-later or commercial | Copyleft clients with a commercial option from Element. Rules for shipping a rebranded build under another name still need to be reviewed against Element’s trademark terms. Element Web · Element X Android · Element X iOS |
| LiveKit | Apache-2.0 | Permissive, with an explicit patent grant. Redistribution needs the license and notices to be kept. Repository |
| MatrixRTC | Specification proposal (MSC4143) | A protocol proposal rather than software: it was merged as a proposal in October 2026 but has not yet been written into the published Matrix specification. Its reference implementation, Element Call, is AGPL-3.0. MSC4143 · Element Call |
| FreeSWITCH | MPL-1.1 | File-level copyleft. SignalWire, founded by FreeSWITCH’s original developers, is its primary sponsor. Carrier and SIP trunk charges are separate from any software license. Repository |
| Keycloak | Apache-2.0 | Permissive. An incubating project of the Cloud Native Computing Foundation, which provides vendor-neutral governance. Repository |
| Samba | GPL-3.0 | Strong copyleft on distribution. Running it as a service carries no network-source obligation, but distributing binaries means providing the corresponding source. License |
| Postfix | EPL-2.0 or IPL-1.0, at the recipient’s choice | Weak copyleft. Confirmed from the LICENSE file in the current official release. Project |
| Rspamd | Apache-2.0 | Permissive. Repository |
| ClamAV | GPL-2.0 | Copyleft on distribution. Maintained by Cisco Talos. Repository |
| Proxmox VE | AGPL-3.0 | Free to use. Proxmox sells subscriptions for access to the enterprise package repository and for support. A no-subscription repository is free, but Proxmox does not recommend it for production servers. For production clusters, the subscription is a realistic line in the operating budget. FAQ · Repositories |
MailThreatZero and the clientst0r integration are not listed: they are separate services rather than components Office Unbound would redistribute, and their terms will be stated if and when they are integrated.
What the license families mean in practice
The same platform will be self-hosted by businesses, hosted by MSPs for many customers, and redistributed under the Office Unbound name. Each license family treats those differently.
Network copyleft (AGPL)
Anyone who modifies the software and lets users interact with it over a network must offer those users the modified source. Running it unmodified, or keeping modifications upstream, keeps this simple. Grommunio, Stalwart, ONLYOFFICE Community, Nextcloud server, Synapse, Element and Proxmox VE are under AGPL.
Copyleft (GPL, MPL, EPL)
Obligations attach to distribution. GPL covers the whole program. MPL, and to a degree EPL, cover the modified files. Hosting a service does not trigger them, but shipping a desktop client or a packaged appliance does, with source to match.
Permissive (Apache-2.0)
Keycloak, LiveKit and Rspamd can be combined and redistributed with few conditions beyond keeping notices. Apache-2.0 also grants patent rights from contributors.
Open-core editions
Stalwart, ONLYOFFICE and Collabora pair an open-source edition with commercial ones, and some features MSPs need, such as multi-tenancy, sit on the commercial side. A deployment may need a commercial license, depending on scale and features.
Trademarks
A license to the code is not a license to the name. Nextcloud, ONLYOFFICE, grommunio and others restrict how their marks appear on modified or rebranded builds. Presenting components under one Office Unbound interface has to respect each set of rules.
Licenses change
Synapse moved from Apache-2.0 to AGPL, and ONLYOFFICE rewrote its additional terms in 2026. Selection weighs who controls a project’s licensing as well as what the license says today.
Office Unbound’s own license
The license for Office Unbound’s own code (the interface, administration and provisioning services, and migration tooling) has not been chosen. It has to be compatible with the components it integrates and suit the way MSPs and customers will deploy it. It will be published with the repository, and no contributions will be accepted before it is.
Whatever is chosen, the platform as a whole will be a mix of licenses, and each component keeps its own. Each Office Unbound release is intended to ship with a full component inventory listing every license, so operators know their obligations without reverse-engineering them.
This is an engineering summary to explain our selection criteria, not legal advice. It reflects the official sources linked above as we found them in November 2026. Where a point is marked as still to be reviewed, we have not found an authoritative answer. Anyone deploying these components commercially should read the licenses themselves and take advice where the stakes justify it.
Know one of these projects well?
If you have run or contributed to any of these components, your experience is useful to the evaluation. Tell us where you could help.