Outlook.com AlternativesOnly Open Source apps categorised as Calendar Apps

The best open source Calendar alternative to Outlook.com is Proton Mail. If that doesn't suit you, our users have ranked more than 50 alternatives to Outlook.com and five of them are open source Calendar Apps so hopefully you can find a suitable replacement. Other interesting open source Calendar App alternatives to Outlook.com are Tuta Mail, Posteo, Infomaniak kMail and EGroupware.

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Alternatives list

  1. Proton Mail icon

    Proton Mail

     1017 likes
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    Proton Mail provides secure email services with end-to-end encryption, ensuring data protection. Swiss-based and outside US jurisdiction, it offers a free version to champion privacy as a human right. Fully accessible on any device.

    98 Proton Mail alternatives

    Cost / License

    • Freemium (Subscription)
    • Open Source

    Application types

    Platforms

    • Mac
    • Windows
    • Linux
    • Online
    • Android
    • iPhone
    • iPad
    • Tor
    • Flathub
    • Snapcraft
    Proton Mail screenshot 1
     
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    25
    Comments about Proton Mail as an Alternative to Outlook.com
    plants
    Positive
    2

    It's not dissimilar to Tutanota. Both these projects are in their infancy, so technically neither is close to a mature product like Outlook, but still worthy of association under the broad category of hosted email-related services

    Oscar Sanderson
    Positive
    0

    I like protonmail because it is private and does not sell your data and is the best thing to switch to if you are using outlook

    Outlook.com Icon
    Proton Mail icon

    • Almost everyone thinks Proton Mail is a great Outlook.com alternative.

    • Proton Mail is the most popular Web-based, Windows, Android & Mac alternative to Outlook.com.

    • Proton Mail is the most popular Open Source & free alternative to Outlook.com.

    • Proton Mail is Freemium and Open SourceOutlook.com is Freemium and Proprietary
    • Proton Mail is Privacy focusedOutlook.com is not according to our users
  2. Tuta Mail icon

    Tuta Mail

     516 likes
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    Offers comprehensive encryption for email, calendar, and contacts with zero personal info required. Built on open source, install via Play Store, F-Droid, or APK, with Tor and VPN support. Both free and paid plans; utilizes quantum-resistant encryption ensuring privacy-focused, anonymous usage.

    103 Tuta Mail alternatives

    Cost / License

    • Freemium (Subscription)
    • Open Source

    Application types

    Platforms

    • Mac
    • Windows
    • Linux
    • Online
    • Android
    • iPhone
    • iPad
    • Flathub
    • F-Droid
    Tuta Mail screenshot 1
     
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    20
  4. Posteo icon

    Posteo

     83 likes
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    Web- and IMAP-based email service. 2GB cost 1€/month. Information is hosted on encrypted hard drives. Electricity from renewable resources is used.

    74 Posteo alternatives

    Cost / License

    • Subscription
    • Open Source

    Application types

    Platforms

    • Online
    Drag and drop
     

    • Posteo is the most popular commercial alternative to Outlook.com.

    • Posteo is Paid and Open SourceOutlook.com is Freemium and Proprietary
    • Posteo is Privacy focused and Security-focusedOutlook.com is not according to our users
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    Screenshot
    Proton Mail icon
    Proton Mail

    Proton Mail provides secure email services with end-to-end encryption, ensuring data protection. Swiss-based and outside US jurisdiction, it offers a free version to champion privacy as a human right. Fully accessible on any devi...

    https://proton.me
    Learn More
  6. Infomaniak kMail icon

    Infomaniak kMail

     38 likes
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    Privacy-focused Swiss email system provides 20 GB free storage, supports custom domains and unlimited storage, syncs contacts and calendars, is ad-free, and is part of a secure productivity suite used by over a million users across Europe on all devices.

    42 Infomaniak kMail alternatives

    Cost / License

    • Freemium (Subscription)
    • Open Source

    Application types

    Platforms

    • Online
    • Android
    • iPhone
    • Android Tablet
    • iPad
    • F-Droid
    Infomaniak kMail screenshot 1
     
  7. EGroupware icon

    EGroupware

     9 likes
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    Web-based team collaboration suite combining email client, calendar, contact and task tools, CRM, project and file management, file sharing, trouble tickets, Rocket.Chat chat, and Collabora online editing. Offers open source, self-hosted, and SaaS options.

    23 EGroupware alternatives

    Cost / License

    • Freemium (Subscription)
    • Open Source

    Application types

    Platforms

    • Linux
    • Online
    • Self-Hosted
    • Software as a Service (SaaS)
    EGroupware on the desktop --- EGroupware offers with the modules - E-mail - Calendar - Address book - InfoLog (tasks, notes, ...) - File manager - Ticket system - Project Manager - Timesheet - Collabora Online Office - Rocket.Chat an entire office in the browser.
     
5 of 5 Outlook.com alternatives