Proton Mail AlternativesOnly Open Source apps categorised as Calendar Apps

The best open source Calendar alternative to Proton Mail is Tuta Mail. If that doesn't suit you, our users have ranked more than 100 alternatives to Proton Mail and four of them are open source Calendar Apps so hopefully you can find a suitable replacement. Other interesting open source Calendar App alternatives to Proton Mail are Posteo, Infomaniak kMail and Kolab Now.

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Proton Mail alternatives are mainly Calendar Apps, but if you're looking for WebMail Providers or Email Clients you can filter on that. Other popular filters include Free, Open Source and Self-Hosted. These are just examples - use the filter bar below to find more specific alternatives to Proton Mail.
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Alternatives list

  1. Tuta Mail icon

    Tuta Mail

     563 likes

    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.

    109 Tuta Mail alternatives

    Cost / License

    Application types

    Platforms

    • Mac
    • Windows
    • Linux
    • Online
    • Android
    • iPhone
    • iPad
    • Flathub
    • F-Droid
    • Android Tablet
    • Thunderbird
    Tuta Mail screenshot 1
     
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    10
  2. Posteo icon

    Posteo

     89 likes

    Web- and IMAP-based email service. 4GB cost 1€/month. Information is hosted on encrypted hard drives. Electricity from renewable resources is used.

    79 Posteo alternatives

    Cost / License

    • Paid
    • Open Source

    Application types

    Platforms

    • Online
    Drag and drop
     
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    1
  4. Infomaniak kMail icon

    Infomaniak kMail

     48 likes

    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.

    50 Infomaniak kMail alternatives

    Cost / License

    Application types

    Platforms

    • Online
    • Android
    • iPhone
    • Android Tablet
    • iPad
    • F-Droid
    Infomaniak kMail screenshot 1
     
  5. Ad
    Screenshot
    Infomaniak kMail icon
    Infomaniak kMail

    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 acro...

    https://www.infomaniak.com
    Learn More
  6. Kolab Now icon

    Kolab Now

     43 likes

    Kolab Now provides a suite of intuitive apps in a user friendly environment to make your day to day online comms easier, smarter and most importantly; secure.

    76 Kolab Now alternatives

    Cost / License

    • Paid
    • Open Source

    Application types

    Platforms

    • Online
    • Software as a Service (SaaS)
    • DAVx5
    Kolab Now email
     
4 of 4 Proton Mail alternatives