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Microsoft Outlook alternatives · Email

Open-source alternatives to Microsoft Outlook

Microsoft Outlook is not described as open source on its own site. 8 of the 51 other email apps in this directory are open source, according to their own sites.

Microsoft Outlook has a free plan. Paid plans start at $1.99 per month, or $19.99 per year (Microsoft 365 Basic). 6 of them are free or have a free plan.

8 tools · Listings checked up to Oct 9, 2026 · Where the facts come from

How each one differs from Microsoft Outlook

  1. Thunderbird

    Free, open-source email app for Windows, macOS, Linux and Android

    • Thunderbird is free. Microsoft Outlook starts at $1.99 per month, or $19.99 per year (Microsoft 365 Basic).
    • Only Thunderbird lists a Linux app.
    • Only Microsoft Outlook lists a web app.
    • Only Microsoft Outlook lists an iOS app.
    Thunderbird vs Microsoft OutlookThunderbird pricing and details

  2. Mailspring

    Open-source desktop email client for Mac, Windows and Linux

    • Mailspring starts at $8 per month, or $85 per year (Mailspring Pro). Microsoft Outlook starts at $1.99 per month, or $19.99 per year (Microsoft 365 Basic).
    • Only Mailspring lists a Linux app.
    • Only Microsoft Outlook lists a web app.
    • Only Microsoft Outlook lists iOS and Android apps.
    Mailspring vs Microsoft OutlookMailspring pricing and details

  3. Wino Mail

    Free, open source mail and calendar app for Windows

    • Only Microsoft Outlook lists a web app.
    • Only Microsoft Outlook lists macOS, iOS and Android apps.
    • Wino Mail is open source, according to its own site.
    • Wino Mail also covers templates.
    Wino Mail vs Microsoft OutlookWino Mail pricing and details

  4. Inbox Zero

    Open source AI email assistant for Gmail and Outlook

    • Inbox Zero starts at $18 per user / month, billed annually ($20 billed monthly). Microsoft Outlook starts at $1.99 per month, or $19.99 per year (Microsoft 365 Basic).
    • Microsoft Outlook has a free plan. Inbox Zero does not, though it has a free trial.
    • Only Inbox Zero lists a browser extension.
    • Inbox Zero is open source and self-hostable, according to its own site.
    Inbox Zero pricing and details

  5. Proton Mail

    End-to-end encrypted email service based in Switzerland

    • Proton Mail starts at $3.99 per month, billed every 12 months ($47.88); $4.99 billed monthly. Microsoft Outlook starts at $1.99 per month, or $19.99 per year (Microsoft 365 Basic).
    • Only Proton Mail lists a Linux app.
    • Proton Mail is open source and end-to-end encrypted, according to its own site.
    • Microsoft Outlook also covers tasks and inbox filtering.
    Proton Mail pricing and details

  6. Tuta Mail

    End-to-end encrypted email and calendar from Germany

    • Tuta Mail starts at €3 per month, paid yearly (lowest paid personal plan). Microsoft Outlook starts at $1.99 per month, or $19.99 per year (Microsoft 365 Basic).
    • Only Tuta Mail lists a Linux app.
    • Tuta Mail is open source and end-to-end encrypted, according to its own site.
    • Microsoft Outlook also covers tasks and inbox filtering.
    Tuta Mail pricing and details

  7. Posteo

    Ad-free, green-powered email from Berlin for €1 a month

    • Posteo starts at €1 per month, prepaid (minimum top-up €12). Microsoft Outlook starts at $1.99 per month, or $19.99 per year (Microsoft 365 Basic).
    • Microsoft Outlook has a free plan. Posteo does not.
    • Only Microsoft Outlook lists Windows, macOS, iOS and Android apps.
    • Posteo is open source and end-to-end encrypted, according to its own site.
    Posteo pricing and details

  8. FairEmail

    Open source, privacy-oriented email client for Android

    • FairEmail starts at $7.49 one-time (pro features, US Google Play in-app purchase). Microsoft Outlook starts at $1.99 per month, or $19.99 per year (Microsoft 365 Basic).
    • Only Microsoft Outlook lists a web app.
    • Only Microsoft Outlook lists Windows, macOS and iOS apps.
    • FairEmail is open source and end-to-end encrypted, according to its own site.
    FairEmail pricing and details

Common questions

Is there an open-source alternative to Microsoft Outlook?

Yes. Thunderbird, Mailspring, Wino Mail, Inbox Zero, Proton Mail, Tuta Mail, Posteo and FairEmail are open source, according to their own sites.

Is Microsoft Outlook open source?

Microsoft Outlook is not described as open source on its own site. The listing was checked on Oct 5, 2026 against the pages it cites.

Which of these email apps are free?

Thunderbird, Mailspring, Wino Mail, Proton Mail, Tuta Mail and FairEmail are free or have a free plan.

When was this checked?

Each listing shows its own date. On this page they run from Oct 4, 2026 to Oct 9, 2026. Vendors change prices and plans without notice, so confirm on their site before you buy.

This page is built from the listings. A tool is here because its listing says so, and the order is how close each is to Microsoft Outlook: the alternatives its listing names first, then the tools that share the most features with it.