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, Betterbird, Evolution and FairEmail.
Cross-platform email client supporting multiple identities, chat, and strong filtering. Extend functionality with add-ons like RSS/Atom feed readers and PGP support.
United States
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.
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.
Germany
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Evolution is a personal information management application that provides integrated mail, calendaring, address book and task list functionality.
United States
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.
NetherlandsEU
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.
United States
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.
GermanyEU
Geary is an email application built around conversations, for the GNOME 3 desktop.
United States
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.
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.
Hong Kong SAR China
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.
Android email app focused on privacy and security, supporting JMAP, IMAP, and SMTP protocols, encrypted credentials, OpenPGP through OpenKeychain, unified inbox, conversation threading, offline mode, strong tracking protection, F-Droid distribution, no ads or Google dependence.
