The best open source alternative to K-9 Mail is Thunderbird. If that doesn't suit you, our users have ranked more than 25 alternatives to K-9 Mail and 15 is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to K-9 Mail are Betterbird, FairEmail, Evolution and Mailspring.
Cross-platform email client supporting multiple identities, chat, and strong filtering. Extend functionality with add-ons like RSS/Atom feed readers and PGP support.
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.
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.
Evolution is a personal information management application that provides integrated mail, calendaring, address book and task list functionality.
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.
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.
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.
No-frills, easy to use, easy to maintain Email client for Android based on the JSON Meta Application Protocol (JMAP).
LucaMail is an open source, cross-platform email client. LucaMail is developed to provide cross-platform alternative to Email Clients that are really exists. LucaMail focuses on modern technologies as well as maintaining a user-friendly UI.
Nylas Mail Lives is a fork of Nylas Mail that fixes many longstanding issues, improves the client experience, and is committed to publish new releases.
Columba is an email client written in Java, featuring a user-friendly graphical interface with wizards and internalionalization support. Its a powerful mail management tool with features to enhance your productivity and communication.
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A really great client used by many, popular because it supports almost every mail provider (not proton ?), and mail providers provide documentation to integrate with Thunderbird. I recommand