The best open source alternative to Spike is Delta Chat. If that doesn't suit you, our users have ranked more than 50 alternatives to Spike and nine of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Spike are Zulip, Mailspring, Claws Mail and Iroco.
Provides a chat experience over email, with all messages and contacts stored on user devices, open source code, group media support, and compatibility with any provider. No central servers, no tracking, works without SIM, and can be used by anyone just via email.
Threaded group conversations combine live and asynchronous messaging, separating topics for organized collaboration, supporting open-source, cloud, and self-hosted deployments, secure communication with encryption, and multi-platform access for diverse teams.
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.
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.
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.
Unofficial, free and open-source Google Inbox Desktop App. Built using electron, Inboxer includes a menu bar which is globalmenu compatible and works, and keyboard shortcuts.
Adcrux is an ESP and SMTP Friendly platform that connects to all your Email Delivery Providers in one place, so you can send across multiple channels.
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The best FOSS alternative to any other chat-styled email clients. I've been hooked ever since I downloaded this application!