The Best Open Source Thunderbird AlternativesTop Email Clients and other similar apps like Thunderbird

The best open source alternative to Thunderbird is K-9 Mail. If that doesn't suit you, our users have ranked more than 50 alternatives to Thunderbird and many of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Thunderbird are Claws Mail, Evolution, Betterbird and SeaMonkey.

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Thunderbird 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, iPhone, Free, Windows + Open Source, Android and Kindle Fire. 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 Thunderbird.
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Alternatives list

  1. K-9 Mail icon

    K-9 Mail

     367 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

    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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    22
    170
    Comments about K-9 Mail as an Alternative to Thunderbird
    Comment summary: K-9 Mail serves as a strong alternative to Thunderbird for Android devices, praised for its OpenKeyChain integration and PGP encryption. It is valued for being open source and user-friendly with options for both POP3 and IMAP. However, criticism centers around its exclusivity to Android, which limits its capability to fully replace desktop-focused Thunderbird for some users.
    Top Positive Comment
    kiminariyamamoto
    7

    Keeps the style of working with email as it should be, like we are used to from proper desktop Mail clients. It's unlike modern fancy clients with couple of buttons without even providing an option to view headers.

    Top Negative Comment
    ByteRanger
    4

    K-9 Mail is not for desktop operating systems, only Android. So you can't replace Thunderbird unless you are also replacing your desktop with a mobile device.

    jondemon
    Negative
    0

    K9 has nothing to do here. It's an android app

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    3 of 22 comments
    Thunderbird Icon
    K-9 Mail icon

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

    • K-9 Mail is the most popular Android & Android Tablet alternative to Thunderbird.

    • K-9 Mail is the most popular Open Source & free alternative to Thunderbird.

    • K-9 Mail is Free and Open SourceThunderbird is also Free and Open Source
    • K-9 Mail is LightweightThunderbird is not according to our users
  2. 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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    8
    51
    Comments about Claws Mail as an Alternative to Thunderbird
    Top Positive Comment
    Jonathan DUMONT
    9

    Despite the style, Claws is super fast and allow you to programs it with Perl and Python which is great for marketing automation.

    Top Negative Comment
    Guest
    19

    No HTML compose is the only reason I don't switch to it. We're not in 1980 anymore.

    Review by a new / low-activity user.
    SViN
    2

    I think most people are using gmail accounts or outlook accounts to access their emails. Claws supports Oauth2 to access said accounts but the implementation, is a complete PAIN.

    I can only realistically recommend something like this if your email provider is not forcing Oauth2.

    I would like for the devs to get to work into making the experience of setting up an account with oauth as painless as thunderbird.

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    3 of 8 comments
    Thunderbird Icon
    Claws Mail icon

    • Most users think Claws Mail is a great Thunderbird alternative.

    • Claws Mail is the most popular Windows & Linux alternative to Thunderbird.

    • Claws Mail is Free and Open SourceThunderbird is also Free and Open Source
    • Claws Mail is LightweightThunderbird 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

    Application types

    Platforms

    • Linux
    • BSD
    • Flathub
    • S/MIME
    • GNOME
    • OpenPGP
    Mail
     
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    5
    71
    Comments about Evolution as an Alternative to Thunderbird
    Top Positive Comment
    Michel Th.
    1

    I use both Thunderbird and Evolution, sometimes Claws-Mail. Evolution is the only one that can show the real "received" date in scams I have been receiving for months. In a today's scam Evolution shows "sent date" (falsified) 14-12-2019 and "received date" 30-01-2023 (evidently true). Thunderbird and Claws-Mail show 14-12-2019 in both columns.

    RemovedUser
    Positive
    0

    As someone who is forced to use M365 at work (but ditching it at home, I let my subscription lapse) this is the closest visually to Outlook. That's important to me. The workflow is essentially identical.

    Review by a new / low-activity user.
    Syntevo GmbH
    1

    Unfortunately, only available on Linux, not on Windows, not on MacOS. Otherwise promising.

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    3 of 5 comments
    Thunderbird Icon
    Evolution icon

    • Most users think Evolution is a great Thunderbird alternative.

    • Evolution is Free and Open SourceThunderbird is also Free and Open Source
  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
     
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    5
    16
    Comments about Betterbird as an Alternative to Thunderbird
    Top Positive Comment
    Sam Lander
    3

    Betterbird does everything that Thunderbird does (it's a soft fork), while improving bugs and features.

    Because it's a soft fork, it retains the security implementations of Thunderbird.

    Thunderbird actually implements many of the bug fixes created by Betterbird, but at a much slower pace.

    drummer_nrg
    Positive
    1

    Betterbird is actively updated by a previous Thunderbird developer and is a fully compatible fork of Thunderbird which fixes some outstanding bugs. My add-ons like keepassxc-mail work fine!

    Guest
    Positive
    2

    It has many fixes for long existing bugs

    Review by a new / low-activity user.
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    3 of 5 comments
    Thunderbird Icon
    Betterbird icon

    • Almost everyone thinks Betterbird is a great Thunderbird alternative.

    • Betterbird is the most popular Mac alternative to Thunderbird.

    • Betterbird is Free and Open SourceThunderbird is also Free and Open Source
    • Betterbird is LightweightThunderbird is not according to our users
  6. SeaMonkey icon

    SeaMonkey

     241 likes
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    Web-browser, advanced e-mail, newsgroup and feed client, IRC chat, and HTML editing in one application. Itis built on the open source Mozilla Gecko engine, the same code which underlies Thunderbird and is the base for the Firefox browser.

    187 SeaMonkey alternatives

    Cost / License

    • Free
    • Open Source

    Application type

    Platforms

    • Mac
    • Windows
    • Linux
    • BSD
    • PortableApps.com
    SeaMonkey screenshot 1
     
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    4
    43
    Comments about SeaMonkey as an Alternative to Thunderbird
    Guest
    Positive
    21

    SeaMonkey is both a WEB BROWSER and EMAIL CLIENT. It's the successor of Mozilla Application Suite or Netscape Communicator, resp. Further it includes also a news client program, an HTML editor (SeaMonkey Composer) and an IRC client (ChatZilla).

    Review by a new / low-activity user.
    apostolfaliagas
    Positive
    15

    It has all of Thunderbird's features and many more.

    Guest
    Negative
    0

    Unfortunately doesn't support synchronization with CardDAV address books.

    Review by a new / low-activity user.
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    3 of 4 comments
    Thunderbird Icon
    SeaMonkey icon

    • Some users think SeaMonkey is a great Thunderbird alternative, some don't.

    • SeaMonkey is Free and Open SourceThunderbird is also Free and Open Source
    • SeaMonkey is LightweightThunderbird is not according to our users
  7. Geary icon

    Geary

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

    Cost / License

    • Free
    • Open Source

    Application type

    Platforms

    • Linux
    • BSD
    • Flathub
    • GNOME
    • Flatpak
    Main window
     
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    4
    34
    Comments about Geary as an Alternative to Thunderbird
    Kelson V
    Positive
    0

    Geary is a lot lighter than Thunderbird, so it's a good choice on lower-end hardware. IMAP features are basic, but 90% of what most people will need. It relies on GNOME for contacts, but the mail app itself can run on any Linux desktop.

    Guest
    Negative
    1

    Doesn't have an address book

    Review by a new / low-activity user.
    Guest
    Negative
    1

    Only IMAP. No support for local folders.

    Review by a new / low-activity user.
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    3 of 4 comments
    Thunderbird Icon
    Geary icon

    • Most users think Geary is a great Thunderbird alternative.

    • Geary is Free and Open SourceThunderbird is also Free and Open Source
    • Geary is LightweightThunderbird is not according to our users
  9. FairEmail icon

    FairEmail

     87 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 (GPL-3.0)

    Application type

    Platforms

    • Android
    • Android Tablet
    • F-Droid
    Compose message view.
    Setup screen.
    Messages view with dark theme.
     
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    2
    12
    Comments about FairEmail as an Alternative to Thunderbird
    ByteRanger
    Negative
    -3

    While I do use FairEmail on my mobile devices and like it, it is not a valid alternative to Thunderbird which is for desktop platforms.

    Guest
    Negative
    -8

    wrong platform. FairEmail is not an alternative for Thunderbird users because it only runs on android. Thunderbird is for desktop operating systems only.

    Review by a new / low-activity user.
    Thunderbird Icon
    FairEmail icon

    • Most users think FairEmail is a great Thunderbird alternative.

    • FairEmail is Freemium and Open SourceThunderbird is Free and Open Source
    • FairEmail is LightweightThunderbird is not according to our users
  10. 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 (GPL-3.0)

    Application type

    Platforms

    • Mac
    • Windows
    • Linux
    • Arch Linux
    • Fedora
    • Ubuntu
    Clean design with integrated search and swipe gestures
     
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    7
    29
    Comments about Mailspring as an Alternative to Thunderbird
    Top Positive Comment
    3psilon
    1

    Great alternative, especially for casual users. Simple, streamlined UI, feels fresh.

    Top Negative Comment
    Guest
    60

    Need to create an account on a third-party company to access your email.

    Review by a new / low-activity user.
    Matt Schaefer
    Negative
    0

    Unable to even find an address book or ability to import a csv file from previous mail client. You lose me when things are made so "modern-clean" that I have to consult a manual or Google things.

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    3 of 7 comments
    Thunderbird Icon
    Mailspring icon

    • Most users think Mailspring is a great Thunderbird alternative.

    • Mailspring is Freemium and Open SourceThunderbird is Free and Open Source
  11. 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).

    42 Sylpheed alternatives

    Cost / License

    • Free
    • Open Source

    Application type

    Alerts

    • Discontinued

    Platforms

    • Mac
    • Windows
    • Linux
    • BSD
    • PortableApps.com
    Main screen
     
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    1
    17
    Comments about Sylpheed as an Alternative to Thunderbird
    Guest
    Negative
    3

    It doesn't encrypt password storage and doesn't have Master Password function

    Review by a new / low-activity user.
    Thunderbird Icon
    Sylpheed icon

    • Most users think Sylpheed is a great Thunderbird alternative.

    • Sylpheed is Free and Open SourceThunderbird is also Free and Open Source
    • Sylpheed is LightweightThunderbird is not according to our users
  12. Mutt icon

    Mutt

     68 likes
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    Mutt is a small but very powerful text-based mail client for Unix operating systems. Some of its features are: color support, message threading, MIME support (including RFC2047 support for encoded headers), PGP/MIME (RFC2015), various features to support mailing lists, including...

    51 Mutt alternatives

    Cost / License

    • Free
    • Open Source

    Application type

    Platforms

    • Mac
    • Windows
    • Linux
    • Xfce
    Mutt screenshot 1
     
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    2
    24
    Comments about Mutt as an Alternative to Thunderbird
    Guest
    Negative
    17

    Thunderbird users want a GUI mail application. Mutt is a command line mail application. They are both good and both popular, but very much apples and oranges.

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

    Mutt's a great email client for your terminal, not a GUI app like Thunderbird.

    Review by a new / low-activity user.
    Thunderbird Icon
    Mutt icon

    • Most users think Mutt is a great Thunderbird alternative.

    • Mutt is Free and Open SourceThunderbird is also Free and Open Source
  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.

    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 Thunderbird.

    • Mailpile is Free and Open SourceThunderbird is also Free and Open Source
  14. 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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    10
12 of 37 Thunderbird alternatives