Zimbra OSE — Open Source Groupware Suite
General Information
Zimbra OSE (Open Source Edition) is the free community build of the Zimbra Collaboration Suite. It’s been around for years as one of the main self-hosted alternatives to Microsoft Exchange. Unlike basic webmail, it includes mail, shared calendars, contacts, and tasks — so it feels closer to groupware than just a mailbox.
A lot of schools, universities, and even municipalities in Europe have adopted it. The reason is simple: no licensing fees, local control, and a web client that users can pick up without training.
How It Works
Zimbra is not just a mail client bolted onto Postfix — it bundles its own stack. Out of the box you get:
– Postfix for SMTP transport,
– OpenLDAP for directory service,
– MariaDB for storage,
– a browser-based client for users.
Mail comes in via SMTP, sits in the database, and is delivered to users through IMAP, POP3, or the web UI. Calendars and contacts sync over CalDAV and CardDAV. TLS, DKIM, SPF, and antivirus/antispam are built in. Authentication can come from Zimbra’s own LDAP or tie into Active Directory.
Functions
| Feature | In practice |
| Platforms | Linux (Ubuntu, RHEL/CentOS most common) |
| Mail protocols | SMTP, IMAP, POP3 |
| Groupware | Mail, calendars, contacts, tasks, shared folders |
| Web client | Rich browser interface with drag-and-drop |
| Security | TLS/SSL, DKIM, SPF, antivirus/antispam |
| Authentication | LDAP, AD integration, SSO possible |
| Extensibility | Zimlets (plugins) for custom features |
| License | GPL open source, community support only |
Installation Guide
1. Prepare a Linux server with plenty of RAM (Zimbra likes memory).
2. Download the OSE installer.
3. Run the setup — it configures Postfix, LDAP, MariaDB, and the web client automatically.
4. Add DNS records (MX, SPF, DKIM) and set up TLS.
5. Create accounts in the admin console.
6. Test mail flow, calendar sync, and web client.
Most admins say the initial install is surprisingly smooth, but tuning for performance takes extra time.
Everyday Use
– Universities use it for shared calendars and mail across thousands of accounts.
– SMBs deploy it as a budget alternative to Exchange.
– Public agencies like it for sovereignty — data stays in their datacenters.
– Internal IT teams run OSE in labs to test groupware setups before moving to paid editions.
For users, the web client looks and works a lot like Outlook Web Access: they can handle mail, book meetings, share calendars, all in one place.
Limitations
– Heavy on resources — needs a decent server with good RAM.
– Some features (mobile ActiveSync, backup tools, clustering) are missing in OSE and only in the commercial edition.
– Updates sometimes trail behind the paid version.
– Admins must plan backup/restore themselves with third-party tools.
Comparison
| Tool | Platforms | Strengths | Best Fit |
| Zimbra OSE | Linux | Full groupware, no license cost | Schools, SMBs, public sector |
| Kopano (Zarafa) | Linux | Outlook integration, modular add-ons | SMEs, government |
| SOGo | Linux | Lightweight groupware, ActiveSync | Mixed device teams |
| Exchange | Windows | Enterprise standard, Outlook native | Large corporates |
| Google Workspace | SaaS | Zero maintenance, collaboration built in | SaaS-first orgs |
Notes from the Field
Admins often joke that Zimbra OSE is “Exchange on a budget.” The feature set is impressive, but it wants hardware — starving it of RAM is the fastest way to get complaints. The web client is solid and users adapt quickly. On the flip side, without the commercial backup tools, recovery has to be handled manually. Still, for environments that want Exchange-like features without license costs, Zimbra OSE continues to be a go-to choice.