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 Windows + Free, Linux, Open Source, Android, Windows + Open Source, Free and iPad. 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

     2610 likes

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

    94 Thunderbird alternatives

    Cost / License

    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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    17
    Thunderbird vs Microsoft Outlook Comments
    Top Positive Comment
    John Fastman
    11

    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.

    Guest
    Negative
    0

    First, it does allow you to connect Zoom for meetings. Second, support is slow or provides little information.

    3 of 17 comments
    • 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

     375 likes

    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.

    45 K-9 Mail alternatives

    Cost / License

    Application type

    Platforms

    • Android
    • Android Tablet
    • F-Droid
    K-9 Mail screenshot 1
     
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    2
    K-9 Mail vs Microsoft Outlook Comments
    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.

    • 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

     211 likes

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

    72 Evolution alternatives

    Cost / License

    Application types

    Platforms

    • Linux
    • BSD
    • Flathub
    • S/MIME
    • GNOME
    • OpenPGP
    Mail
     
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    6
    Evolution vs Microsoft Outlook Comments
    Top Positive Comment
    Guest
    2

    Fast and User friendly. Light-weight and classic menus really give a touch of old classic office 2003. Easy integration of mails especially exchange mails without facing any bottlenecks.

    gifford
    Positive
    -1

    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

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

    3 of 6 comments
    • Evolution is Free and Open SourceMicrosoft Outlook is Paid and Proprietary
  5. Betterbird icon

    Betterbird

     88 likes

    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.

    35 Betterbird alternatives

    Cost / License

    • Free
    • Open Source

    Application type

    Platforms

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

    Mailspring

     118 likes

    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.

    49 Mailspring alternatives

    Cost / License

    Application type

    Platforms

    • Mac
    • Windows
    • Linux
    • Arch Linux
    • Fedora
    • Ubuntu
    Clean design with integrated search and swipe gestures
     
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    3
    Mailspring vs Microsoft Outlook Comments
    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!

    Guest
    Positive
    0

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

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

    FairEmail

     94 likes

    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.

    46 FairEmail alternatives

    Cost / License

    Application type

    Platforms

    • Android
    • Android Tablet
    • F-Droid
    Compose message view.
     
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    1
    FairEmail vs Microsoft Outlook Comments
    Rasheed AZ
    Positive
    1

    open source, secure, regularly updated

    • 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

     109 likes

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

    48 Geary alternatives

    Cost / License

    • Free
    • Open Source

    Application type

    Platforms

    • Linux
    • BSD
    • Flathub
    • GNOME
    • Flatpak
    • Linux Mobile
    Main window
     
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    1
    Geary vs Microsoft Outlook Comments
    Guest
    Positive
    1

    Beatiful frontend Stable working Advanced features

    • 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

     105 likes

    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.

    68 Claws Mail alternatives

    Cost / License

    • Free
    • Open Source

    Application types

    Platforms

    • Windows
    • Linux
    • BSD
    The main window
     
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    1
    Claws Mail vs Microsoft Outlook Comments
    Guest
    Negative
    -2

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

    • 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
  11. Pelton icon

    Pelton

     12 likes

    Free, open-source client for Windows, macOS, and Linux, offering local email storage and search online or offline, privacy-focused with no telemetry or third-party servers, IMAP and SMTP support, intuitive interface, and multiple account support.

    18 Pelton alternatives

    Cost / License

    Application type

    Platforms

    • Mac
    • Windows
    • Linux
    Pelton screenshot 1
     
  12. Aerion icon

    Aerion

     10 likes

    Modern open-source email client supporting unified inbox, IMAP/SMTP, Gmail and Outlook with OAuth2, conversation threading, rich text TipTap composing, CardDAV contact sync, tracking removal, dark mode, color themes, vim-style shortcuts, and a detachable window.

    49 Aerion alternatives

    Cost / License

    Application type

    Platforms

    • Mac
    • Windows
    • Linux
    • Flathub
    • Flatpak
    Aerion screenshot 1
     
  13. KMail icon

    KMail

     49 likes

    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.

    54 KMail alternatives

    Cost / License

    • Free
    • Open Source

    Application type

    Platforms

    • Linux
    • BSD
    KMail screenshot 1
     
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    1
    KMail vs Microsoft Outlook Comments
    Guest
    Positive
    1

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

    • KMail is Free and Open SourceMicrosoft Outlook is Paid and Proprietary
    • KMail is Privacy focused and LightweightMicrosoft Outlook is not according to our users
  14. Kontact icon

    Kontact

     37 likes

    Personal Information Manager (Mail, Contact, Calendar and more).

    Cost / License

    • Free
    • Open Source

    Application types

    Platforms

    • Linux
    • Flathub
    Kontact screenshot 1
     
12 of 39 Microsoft Outlook alternatives