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 50 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, Self-Hosted and Open Source. 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

     531 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.

    103 Tuta Mail alternatives

    Cost / License

    Application types

    Platforms

    • Mac
    • Windows
    • Linux
    • Online
    • Android
    • iPhone
    • iPad
    • Flathub
    • F-Droid
    Tuta Mail screenshot 1
     
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    10
    Tuta Mail vs Proton Mail Comments
    Top Positive Comment
    Advik
    3

    Tutanota is end-to-end encrypted, as well offers 1 GB mail storage on free plan. The pro plans are cheap compared to protonmail, so it's better for low budget users

    Conner Vieira
    Positive
    -2

    Tutanota has historically been a lot more transparent, and arguably has been more honest about issues and plans

    Ada Byron
    Positive
    1

    I use both, ProtonMail is more secure but also more expensive and not as user-friendly. You cannot beat the price of Tutanota for service they provide.

    3 of 10 comments
    • Tuta Mail is Freemium and Open SourceProton Mail is also Freemium and Open Source
    • Tuta Mail is LightweightProton Mail is not according to our users
  2. Posteo icon

    Posteo

     85 likes

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

    73 Posteo alternatives

    Cost / License

    • Paid
    • Open Source

    Application types

    Platforms

    • Online
    Drag and drop
     
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    1
    Posteo vs Proton Mail Comments
    John Fastman
    Positive
    2

    Posteo could be a good Protonmail alternative. It offers email encryption in a similar way: Posteo's people, government surveillance and successful hackers won't get into your email content. It also offers a calendar feature, which protonmail users have been crying out for for years.

    • Posteo is Paid and Open SourceProton Mail is Freemium and Open Source
    • Posteo is Security-focusedProton Mail is not according to our users
  4. Infomaniak kMail icon

    Infomaniak kMail

     43 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.

    44 Infomaniak kMail alternatives

    Cost / License

    Application types

    Platforms

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

    Cost / License

    • Paid
    • Open Source

    Application types

    Platforms

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