Open-source Spark Mail alternatives

Mailspring is a desktop email client for Mac, Windows and Linux, built as a new version of Nylas Mail by one of its original authors. The interface is written in TypeScript with Electron and React, and the mail sync engine is written in C++ and runs locally on your computer. Both parts are open source under GPLv3.

Features include a unified inbox, snooze, send later, mail rules and templates, and a plugin architecture makes the app easy to extend. The README states that these features run in the client and that email credentials are not sent to the cloud. The IMAP-based sync engine replaces the earlier JavaScript sync code used by Nylas Mail.

A paid Mailspring Pro subscription unlocks extra features for people who send a lot of email, such as link tracking, read receipts, mailbox analytics and contact and company profiles. Compiled downloads are offered for Windows, macOS and Linux, including deb, rpm and snap packages.

Thunderbird for Android is an open-source email app for Android that manages multiple email accounts from one place, with an optional Unified Inbox. It is the Android edition of Thunderbird, developed by a team of developers with help from a community of volunteers, and it is based on K-9 Mail, formerly the project's name. It is written in Kotlin.

The project states that Thunderbird never treats private data as a product. The app can be installed from Google Play or F-Droid, in stable or beta versions, from GitHub releases, through FFUpdater, or with Obtainium. The beta channel gives early access to features still under development, and signing certificate fingerprints are published in the security document.

Help comes from a knowledge base, a support forum and a GitHub issue tracker, and feature ideas are discussed on connect.mozilla.org. Release notes describe what changed in each version. The app is released under the Apache 2.0 license and runs on the phone itself, connecting to the email providers you configure.

Inbox Zero is an AI assistant for email, built to cut the time people spend in their inbox. It organizes messages, pre-drafts replies in the user's tone and style, manages the calendar and files attachments. It can also be chatted with from Slack or Telegram, and it is positioned as an open-source alternative to Fyxer that is more customizable and secure.

Its feature set includes AI rules written in plain English, Reply Zero to track emails awaiting a response, a bulk unsubscriber and bulk archiver, a cold email blocker, email analytics and meeting briefs that pull context from email and calendar. Smart filing saves attachments to Google Drive or OneDrive, and the project topics point to Gmail integration.

The app is built with Next.js, Tailwind CSS, shadcn/ui, Prisma and PostgreSQL in a Turborepo monorepo, with Upstash used for supporting services. A hosted version runs at getinboxzero.com, and the repository includes instructions for self-hosting. Feature requests go through GitHub issues or Discord, and the license is specified in the repository.

FairEmail is an open-source email app for Android that is designed around privacy and a deliberately minimal interface. It is an email client only, so you bring your own address, and it works with virtually all providers, including Gmail, Outlook and Yahoo, using standard protocols such as IMAP, POP3 and SMTP.

The app supports unlimited accounts and addresses, a unified inbox, conversation threading, two-way synchronization, push notifications and offline storage and operations. Privacy features include OpenPGP and S/MIME encryption, a safe message view that strips unsafe HTML and scripting, confirmation before showing images or opening links, detection of tracking images, and warnings when a message cannot be authenticated.

FairEmail is written in Java, licensed under GPL-3.0 and provided by FairCode B.V. It does not store data on third-party servers. Non-standard protocols such as Microsoft Exchange Web Services and ActiveSync are not supported, and the app runs a foreground service with a low-priority notification so new mail is not missed.

Mailpile is a modern web-mail client that aims to make private email easier, with user-friendly encryption and an emphasis on a clean interface. It was designed to make receiving and sending PGP-encrypted or signed email convenient, and its development was funded by a large community of backers.

Its core is a custom-written search engine built to handle large volumes of email on consumer hardware. Mail is organized with tags, similar to Gmail labels, and incoming messages can be tagged automatically through static rules or Bayesian classifiers. The primary interface is web-based, with a basic command-line interface and an API for developers, so it can run as a local desktop application or as web-mail on a personal server.

The README now carries a notice that development on this codebase has halted until the Python 3 rewrite is complete, and it points to legacy Docker images for anyone who needs to open version 1 data. The repository metadata lists the license as Other, while the project says its code is released under an OSI-approved free software license.