There are many alternatives to K-9 Mail for Android if you are looking for a replacement. The best open source Android alternative is Thunderbird. If that doesn't suit you, our users have ranked more than 25 alternatives to K-9 Mail and seven of them are open source and available for Android so hopefully you can find a suitable replacement. Other interesting open source Android alternatives to K-9 Mail are FairEmail, Infomaniak kMail, Ltt.rs and Librem Mail.
Cross-platform email client supporting multiple identities, chat, and strong filtering. Extend functionality with add-ons like RSS/Atom feed readers and PGP support.
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.
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.
No-frills, easy to use, easy to maintain Email client for Android based on the JSON Meta Application Protocol (JMAP).
An open-source email app for Android delivering privacy with a user-friendly minimalist design. It supports multiple accounts, a unified inbox, conversation view, and encryption. Offers two-way sync, offline storage, minimal data use, and battery efficiency. Export settings for easy setup.
United States
Netherlands
EU
Switzerland
Germany
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