The Best Open Source Microsoft Outlook AlternativesTop Email Clients & Calendar Apps like Microsoft Outlook

The best open source alternative to Microsoft Outlook is Thunderbird. If that doesn't suit you, our users have ranked more than 100 alternatives to Microsoft Outlook and many of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Microsoft Outlook are K-9 Mail, Evolution, Betterbird and Mailspring.

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Microsoft Outlook alternatives are mainly Email Clients, but if you're looking for Calendar Apps or Contact Managers you can filter on that. Other popular filters include Linux, Open Source, Android, Windows + Free, Windows, Windows + Open Source and EU. You can also filter by region, for example EU-based alternatives if you prefer software developed in the European Union. These are just examples - use the filter bar below to find more specific alternatives to Microsoft Outlook.
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Alternatives list

  1. Thunderbird icon

    Thunderbird

     2522 likes
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    Cross-platform email client supporting multiple identities, chat, and strong filtering. Extend functionality with add-ons like RSS/Atom feed readers and PGP support.

    87 Thunderbird alternatives

    Cost / License

    • Free
    • Open Source

    Application types

    Platforms

    • Mac
    • Windows
    • Linux
    • Android
    • BSD
    • AppImageHub
    • Snapcraft
    • Flathub
    • PortableApps.com
    • F-Droid
    • Haiku
    • Homebrew
    • Chocolatey
    Thunderbird screenshot 1
     
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    15
    192
    Comments about Thunderbird as an Alternative to Microsoft Outlook
    Comment summary: Thunderbird is praised for being a strong open-source alternative to Microsoft Outlook, with numerous extensions enhancing its functionality. Users value its seamless compatibility across different Linux distributions and appreciate the built-in calendar, though some desire further improvements in calendar integration. Many find Thunderbird intuitive and easier to use than Outlook, though a minority consider its interface complicated. Security and customization are notable advantages.
    Top Positive Comment
    John Fastman
    12

    Despite its age, Thunderbird remains the most fully featured email client out there. Various add-ons can extend its functionality, it (now) has a built in calendar and, unlike Outlook, it's free, open source and respects your privacy.

    Top Negative Comment
    Guest
    15

    The interface is just impossible for many people to use.

    Review by a new / low-activity user.
    Shaz Shah
    Positive
    0

    I remember when I first used Thunderbird, many years ago, as I'd rather have a single interface for productivity. But the interface through me off and I never got on with the editor. Coming back to it now, it's fresher and smoother. Absolutely love it. In my opinion this is the only true alternative to Outlook. Then there's the bonus of extensions to further enhance it. Though, I'd like to see further improvements to the calendar integration.

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    3 of 15 comments
    Microsoft Outlook Icon
    Thunderbird icon

    • Most users think Thunderbird is a great Microsoft Outlook alternative.

    • Thunderbird is the most popular Windows, Android, Mac & Linux alternative to Microsoft Outlook.

    • Thunderbird is the most popular Open Source & free alternative to Microsoft Outlook.

    • Thunderbird is Free and Open SourceMicrosoft Outlook is Paid and Proprietary
    • Thunderbird is Customizable and Privacy focusedMicrosoft Outlook is not according to our users
  2. K-9 Mail icon

    K-9 Mail

     366 likes
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    K-9 is an open source email client for Android with multi-folder sync, email signatures, Bcc-to-self, return-address configuration, keyboard shortcuts, Exchange Support, message flagging, IMAP deletes, saving attachments, configurable notifications and more.

    42 K-9 Mail alternatives

    Cost / License

    • Free
    • Open Source

    Application type

    Platforms

    • Android
    • Android Tablet
    • F-Droid
    K-9 Mail screenshot 1
    K-9 Mail screenshot 2
    K-9 Mail screenshot 3
     
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    2
    21
    Comments about K-9 Mail as an Alternative to Microsoft Outlook
    John Fastman
    Positive
    4

    Not the prettiest, but it's open source (trustworthy), free, privacy-respecting and very functional. My go-to Android email client. I cannot hesitate less in recommending it.

    Online_Purchases1
    Positive
    1

    Deletes e-mail from the server even when underground in NYC subway. Makes life so easy to delete e-mails while on the train When surfacing, K-9 syncs with the server and POOF....those unwanted e-mails are gone from the server.

    Review by a new / low-activity user.
    Microsoft Outlook Icon
    K-9 Mail icon

    • Most users think K-9 Mail is a great Microsoft Outlook alternative.

    • K-9 Mail is the most popular Android Tablet alternative to Microsoft Outlook.

    • K-9 Mail is Free and Open SourceMicrosoft Outlook is Paid and Proprietary
    • K-9 Mail is Privacy focused and LightweightMicrosoft Outlook is not according to our users
  4. Evolution icon

    Evolution

     205 likes
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    Evolution is a personal information management application that provides integrated mail, calendaring, address book and task list functionality.

    69 Evolution alternatives

    Cost / License

    • Free
    • Open Source

    Application types

    Platforms

    • Linux
    • BSD
    • Flathub
    • S/MIME
    • GNOME
    • OpenPGP
    Mail
     
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    6
    45
    Comments about Evolution as an Alternative to Microsoft Outlook
    Top Positive Comment
    Guest
    2

    It runs on Linux and you still have content if you are offline.

    Review by a new / low-activity user.
    gifford
    Positive
    0

    I can just find everything instantly, Outlook I'm having to go to Chat GPT to research where the option is hidden!

    Guest
    Positive
    -1

    This would be the best alternative, except that it requires the Gnome desktop, so users of other desktop environments need to look elsewhere.

    Review by a new / low-activity user.
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    3 of 6 comments
    Microsoft Outlook Icon
    Evolution icon

    • Most users think Evolution is a great Microsoft Outlook alternative.

    • Evolution is Free and Open SourceMicrosoft Outlook is Paid and Proprietary
  5. Betterbird icon

    Betterbird

     70 likes
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    Betterbird, a soft fork of Mozilla Thunderbird, improves user experience with advanced features, bug fixes, security patches, and multi-row descriptions similar to Outlook. Compatible with multiple platforms and can be used alongside Thunderbird, sharing the same profile.

    28 Betterbird alternatives

    Cost / License

    • Free
    • Open Source

    Application type

    Platforms

    • Mac
    • Windows
    • Linux
    • Flathub
    • Chocolatey
    Betterbird screenshot 1
     
  6. Mailspring icon

    Mailspring

     113 likes
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    Boost your productivity and send better email with Mailspring, a beautiful, fast email client for Mac, Windows, and Linux. Mailspring is free to use and an optional pro version adds read receipts, send later, reminders and more.

    43 Mailspring alternatives

    Cost / License

    • Freemium (Subscription)
    • Open Source

    Application type

    Platforms

    • Mac
    • Windows
    • Linux
    • Arch Linux
    • Fedora
    • Ubuntu
    Clean design with integrated search and swipe gestures
     
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    3
    21
    Comments about Mailspring as an Alternative to Microsoft Outlook
    Paul
    Positive
    0

    Since Thunderbird is still bloated and overcomplicated for me on Linux, I'm using Mailspring for the moment. It's good, the interface is nice and it has plenty of features.

    Guest
    Positive
    0

    Beautiful theme options with custom designed themes available. I tried thunderbird a few times but always came back to mailspring. I LOVE the "views" feature that shows you how many times an email was opened and if it was forwarded by the recipient!

    Review by a new / low-activity user.
    Guest
    Positive
    0

    Speed of work, good connection to the mail server, speed search.

    Review by a new / low-activity user.
    Microsoft Outlook Icon
    Mailspring icon

    • Most users think Mailspring is a great Microsoft Outlook alternative.

    • Mailspring is Freemium and Open SourceMicrosoft Outlook is Paid and Proprietary
    • Mailspring is CustomizableMicrosoft Outlook is not according to our users
  7. FairEmail icon

    FairEmail

     86 likes
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    Open-source, privacy-focused email client offering features such as encryption via OpenPGP and S/MIME, unlimited accounts, a unified inbox, customizable settings, and reformatted emails to prevent phishing. Supports two-way synchronization and provides Pro features for additional customization.

    43 FairEmail alternatives

    Cost / License

    • Freemium (Pay once)
    • Open Source

    Application type

    Platforms

    • Android
    • Android Tablet
    • F-Droid
    Compose message view.
    Setup screen.
    Messages view with dark theme.
     
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    1
    Comments about FairEmail as an Alternative to Microsoft Outlook
    Rasheed AZ
    Positive
    1

    open source, secure, regularly updated

    Microsoft Outlook Icon
    FairEmail icon
    • FairEmail is Freemium and Open SourceMicrosoft Outlook is Paid and Proprietary
    • FairEmail is Privacy focused, Customizable and LightweightMicrosoft Outlook is not according to our users
  9. Geary icon

    Geary

     105 likes
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    Geary is an email application built around conversations, for the GNOME 3 desktop.

    46 Geary alternatives

    Cost / License

    • Free
    • Open Source

    Application type

    Platforms

    • Linux
    • BSD
    • Flathub
    • GNOME
    • Flatpak
    Main window
     
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    1
    12
    Comments about Geary as an Alternative to Microsoft Outlook
    Guest
    Positive
    1

    Beatiful frontend Stable working Advanced features

    Review by a new / low-activity user.
    Microsoft Outlook Icon
    Geary icon

    • Most users think Geary is a great Microsoft Outlook alternative.

    • Geary is Free and Open SourceMicrosoft Outlook is Paid and Proprietary
    • Geary is Lightweight and Privacy focusedMicrosoft Outlook is not according to our users
  10. Claws Mail icon

    Claws Mail

     106 likes
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    Email and news client on GTK+ foundation featuring quick performance, extensible plugins, secure protocols, advanced filtering, strong folder and contact management, lightweight design, powerful search, customizable interface, multi-language support, and encryption.

    62 Claws Mail alternatives

    Cost / License

    • Free
    • Open Source

    Application types

    Platforms

    • Windows
    • Linux
    • BSD
    The main window
     
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    1
    Comments about Claws Mail as an Alternative to Microsoft Outlook
    Guest
    Negative
    -2

    Claws Mail is email only, does not have many of the features of Outlook. Displaying HTML mail is sketchy at best.

    Review by a new / low-activity user.
    Microsoft Outlook Icon
    Claws Mail icon

    • Some users think Claws Mail is a great Microsoft Outlook alternative, some don't.

    • Claws Mail is Free and Open SourceMicrosoft Outlook is Paid and Proprietary
    • Claws Mail is Lightweight, Customizable and Privacy focusedMicrosoft Outlook is not according to our users
  12. KMail icon

    KMail

     49 likes
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    KMail is the email component of Kontact, the integrated personal information manager of KDE. Kontact is a personal information manager and groupware software suite for KDE. It supports calendars, contacts, notes, to-do lists, news, and email.

    52 KMail alternatives

    Cost / License

    • Free
    • Open Source

    Application type

    Platforms

    • Linux
    • BSD
    KMail screenshot 1
     
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    1
    12
    Comments about KMail as an Alternative to Microsoft Outlook
    Guest
    Positive
    1

    This is a very integrated suite of programs offering all the functionality of Outlook. Native to KDE's Plasma desktop environment.

    Review by a new / low-activity user.
    Microsoft Outlook Icon
    KMail icon

    • Almost everyone thinks KMail is a great Microsoft Outlook alternative.

    • KMail is Free and Open SourceMicrosoft Outlook is Paid and Proprietary
    • KMail is Privacy focusedMicrosoft Outlook is not according to our users
  13. Mailpile icon

    Mailpile

     117 likes
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    Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features. The development of Mailpile is funded by a large community of backers and all code related to the project is and will be released under an OSI approved Free Software license.

    59 Mailpile alternatives

    Cost / License

    • Free
    • Open Source

    Application types

    Alerts

    • Discontinued

    Platforms

    • Linux
    • Online
    • Self-Hosted
    • OpenPGP
    Mailpile screenshot 1
     

    • Mailpile is the most popular Self-Hosted alternative to Microsoft Outlook.

    • Mailpile is Free and Open SourceMicrosoft Outlook is Paid and Proprietary
  14. Sylpheed icon

    Sylpheed

     62 likes
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    Sylpheed is a simple, lightweight but featureful, and easy-to-use e-mail client (mailer, MUA).

    Cost / License

    • Free
    • Open Source

    Application type

    Alerts

    • Discontinued

    Platforms

    • Mac
    • Windows
    • Linux
    • BSD
    • PortableApps.com
    Main screen
     
12 of 36 Microsoft Outlook alternatives