The best Calendar alternative to K-9 Mail is Thunderbird, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to K-9 Mail and 11 are Calendar Apps so hopefully you can find a suitable replacement. Other interesting Calendar App alternatives to K-9 Mail are Betterbird, Evolution, Microsoft Outlook and Spark 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.
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.
Evolution is a personal information management application that provides integrated mail, calendaring, address book and task list functionality.
Streamline emails, calendars, contacts, and task management with a smart inbox prioritizing critical messages. Includes calendar scheduling, third-party app integration like Facebook, Dropbox, and Trello, and support for Microsoft Exchange, Outlook.com, and Gmail, ensuring efficient task and email handling.
E-mail app with all the great features: swipes (long/short), snooze, add (multiple) attachments from the cloud, support for various e-mail services 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.
A secure, cross-platform email client with a unified inbox, built-in PGP encryption, and optional AI. Use powerful AI when you want it or turn it off completely. Your email, your control.
OfficeMail Pro, an email client app using ActiveSync is not only a secure and safe email client but also an app reinforcing a various convenience aspect. It is a product that has been significantly improved and implemented plenty of features like the shared mailbox and calendars...
Koma-Mail is an email client which is comfortable and has very handy functions. It does not need an installation, so that you can run Koma-Mail from your flash drive but, of course, also on your desktop.
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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