There are three ways to replace Outlook at work. Thunderbird, Mozilla's free software email client, is installed on computers. A webmail such as Zimbra or SOGo runs in the browser. Outlook can also stay, connected over IMAP to a European email provider. The choice mostly depends on shared calendars and on how you manage your computers.
The three options at a glance
| Thunderbird | Webmail (Zimbra, SOGo, Roundcube) | Outlook over IMAP | |
|---|---|---|---|
| IMAP with any provider, Exchange through EWS or Microsoft Graph | In the browser, nothing installed on computers | IMAP | |
| Shared calendars and contacts | CalDAV and CardDAV | Built into Zimbra and SOGo | No CalDAV without an add-in |
| Fleet management | MSI, policies.json, ADMX templates for GPOs | Nothing on computers | Office settings through GPO |
| Licence | Free software (MPL 2.0), no cost | Free software, included in the email subscription | Microsoft licence per user |
| Systems | Windows, macOS, Linux | Any browser | Windows, macOS |
Thunderbird
Thunderbird reads mail over IMAP, handles calendars over CalDAV and contacts over CardDAV. It runs on Windows, macOS and Linux, and is listed in the French interministerial free software catalogue (SILL), the list of free software recommended to public bodies.
At work, it is deployed like any managed application. The official MSI package installs silently through WAPT, a GPO, SCCM or Intune. A policies.json file, the same on Windows, macOS and Linux, enforces add-ons, updates and shared tags. The ESR, with one major version a year, suits a fleet that depends on add-ons such as Quicktext.
Thunderbird also connects to Exchange without an add-on since version 153 ESR (July 2026), and to Microsoft 365 through Microsoft Graph since version 154 (August 2026), for email only. Our page Thunderbird for business covers deployment and configuration.
A webmail
A webmail needs nothing on computers: users open their browser and sign in. It is the simplest option for a mixed fleet or shared computers.
- Zimbra combines email, shared calendars, contacts and tasks. OVHcloud offers it as a hosted service, and the French National Centre for Scientific Research (CNRS) moved close to 50,000 mailboxes to it.
- SOGo adds calendars and contacts that sync with phones.
- Roundcube is a lightweight webmail focused on email.
- Messages, the La Suite numérique webmail, is offered to small French municipalities by the national territorial cohesion agency (ANCT).
Webmail and Thunderbird can be used together: the same mailbox can be read in both.
Keeping Outlook, connected over IMAP
Outlook reads and sends mail over IMAP with most providers. You can leave Exchange Online and keep Outlook during the cut-over. Classic Outlook does not sync CalDAV calendars without an add-in, and its licence still has to be paid. Shared calendars then go through the webmail.
On phones
The Android or iOS mail app connects over IMAP, and Zimbra Pro adds ActiveSync for mail, calendars and contacts. Thunderbird has also been available on Android since October 2024, built from the K-9 Mail code.
How to choose
| Your situation | Suggested option |
|---|---|
| A device management tool and shared reply templates | Thunderbird, ESR version |
| Shared computers, or several operating systems | Webmail, plus Thunderbird for those who want it |
| Leaving Exchange Online step by step | Outlook over IMAP during the cut-over, then Thunderbird or webmail |
flowchart LR A["Exchange Online<br/>and Outlook"] --> B["Hosted email in Europe<br/>same addresses"] B --> C["Thunderbird<br/>on computers"] B --> D["Webmail<br/>Zimbra or SOGo"] B --> E["Phones<br/>IMAP or ActiveSync"]
Frequently asked questions
What is the best free alternative to Outlook?
Thunderbird is the most widely used free and open source email client. It handles email, calendars and contacts, and can be deployed across a fleet with shared settings.
Does Thunderbird work with Exchange and Microsoft 365?
For email, yes. Version 153 ESR uses Exchange Web Services (EWS), which Microsoft has blocked by default in Exchange Online since 1 October 2026. The Release channel uses Microsoft Graph since version 154. Microsoft 365 calendars and contacts are not supported yet.
Can you keep Outlook when leaving Microsoft 365?
Yes for email, over IMAP. Shared calendars then go through the webmail or Thunderbird, and the Outlook licence still has to be paid.
Can Outlook PST archives be recovered?
Yes. Thunderbird's import tool reads data from an Outlook installed on the same computer. Without Outlook, PST files are converted to mbox with readpst, a free software tool. Our guide Migrating from Outlook to Thunderbird details the steps.
Hebeko can help
Hebeko migrates the email of small businesses, non-profits and local authorities: mailboxes moved with the same addresses, Thunderbird deployed with the same settings on every computer, Outlook archives recovered, user training.
- Thunderbird for business
- Replacing Exchange and Outlook
- Outlook and Exchange Online outage of 12 and 13 November 2026
- Moving from Microsoft 365 to free software
To choose the right option for your computers, write to us.
Sources
- Thunderbird 153 : nouvelle version ESR, support Exchange natif et 61 failles corrigées, IT-Connect, 24 July 2026
- Thunderbird 154 Adds Microsoft Graph Support for Microsoft 365, IT-Connect, August 2026
- Exchange Online EWS, Your Time is Almost Up, Microsoft, 5 February 2026, updated 27 October 2026
- Thunderbird@Work, Thunderbird documentation for organisations
- Socle interministériel de logiciels libres, DINUM
- Thunderbird for Android finally officially released, Gigazine, 31 October 2024