Open-source Microsoft Outlook 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.

Stalwart is an open-source mail and collaboration server written in Rust and designed to be secure, fast, robust and scalable. It supports JMAP, IMAP4, POP3, SMTP, CalDAV, CardDAV and WebDAV, so a single server can handle email, calendars, contacts and file storage.

On the email side it offers JMAP for Mail and Sieve, IMAP4rev2 and IMAP4rev1 with ManageSieve, and POP3. The SMTP server includes DMARC, DKIM, SPF and ARC for message authentication, transport security through DANE, MTA-STS and TLS reporting, automated DKIM key rotation, throttling and filtering rules, sieve scripting and milter integration, and distributed virtual queues.

The collaboration features cover CalDAV and JMAP for Calendars, CardDAV and JMAP for Contacts, and WebDAV and JMAP for File Storage, with fine-grained sharing through WebDAV ACL and JMAP Sharing. A built-in spam and phishing filter includes a rule set and LLM-driven message analysis. It is self-hosted software that administrators run on their own servers.

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.

mailcow: dockerized is an open-source mail server package that runs as a set of Docker containers managed with Docker Compose. It bundles the pieces needed to run your own email, so administrators do not have to assemble and configure each component separately.

The stack is built around established open-source software: Postfix for SMTP, Dovecot for IMAP, Rspamd for spam filtering, ClamAV for antivirus scanning, SOGo for groupware and webmail, and ACME support for certificates. A web administration interface sits on top. Topics in the repository also mention the Olefy component for document scanning.

mailcow is released under the GNU General Public License v3 and installed on your own server following the official documentation. It is managed and maintained by The Infrastructure Company GmbH, and the project offers support contracts through Servercow as well as a one-time support payment option. The community gathers on Telegram, Mastodon and GitHub Sponsors.

Tuta, formerly known as Tutanota, is an email service built around security and privacy. It uses built-in end-to-end encryption to protect emails, contacts and calendar entries across devices, and it lets users communicate securely with other people regardless of what provider they use.

The open-source repository contains the client applications, which include a web client, an iOS app and a desktop client, along with separate calendar releases for mobile platforms. The code is written in TypeScript and JavaScript on top of the Mithril framework and is licensed under GPL-3.0. Build, translation and development instructions are provided in dedicated files, and the roadmap and feature requests are handled through Reddit and the vendor website.

Tuta is a hosted service operated by the vendor rather than a product you install on your own server. The repository states that it does not accept contributions or bug reports produced with the help of large language models, and maintainers close such submissions without explanation.

Mailu is a full-featured mail server delivered as a set of Docker images. The project aims to give people an easily set up and maintained mail server made entirely of free software, without proprietary components or unrelated groupware features and without trackers.

It covers the standard email server functions, IMAP and SMTP with client auto-configuration profiles, along with aliases, domain aliases, custom routing and full-text search of attachments. Users get webmail, auto-reply, auto-forward and managesieve, while admins get global admin roles, per-domain delegation, announcements and quotas. Security features include enforced TLS, DANE, MTA-STS, Let's Encrypt certificates, outgoing DKIM and an antivirus scanner, and antispam features include greylisting, auto-learning, DMARC, SPF and anti-spoofing checks.

Mailu is written in Python and deployed with Docker or Docker Compose. The project's own configuration files, Dockerfiles and code are MIT-licensed, and all bundled components are free software compatible with it. A demo server and website documentation are provided.

Roundcube Webmail is a multilingual IMAP client that runs in the browser and behaves like a desktop application. It offers the functions people expect from an email client, such as MIME support, an address book, folder management, message search and spell checking, without installing anything on their own computers.

It is written in PHP and JavaScript and requires MariaDB, MySQL, PostgreSQL or SQLite as its database. A plugin API makes it extensible, and the user interface can be customized with skins. The client uses jQuery 3.x and inherits its browser support, covering current versions of Chrome, Edge, Firefox and Safari. Releases follow semantic versioning, and the README ships separate install and upgrade guides.

Roundcube is licensed under GPL-3.0 and is installed on a web server that connects to an existing IMAP mail server. Note that the repository README warns that the Git snapshot is not a stable version and should not replace a production installation.

Webclients is the monorepo that holds the source for the Proton web applications. It includes the apps themselves, their dependencies, shared modules and the tooling used to develop them, covering products such as Proton Mail, Calendar, Drive, Account, VPN, Pass, Wallet, Lumo and Meet.

The monorepo is based on pnpm workspaces with unified versioning across all packages, and the apps are written in TypeScript with React. The README explains the prerequisites for working on the code, how to version an application manually, and how the VPN app differs because it is served from two separate domains with their own entry points.

The code is published under GPL-3.0 and the project invites community help with translations. Because it contains client-side applications for Proton's own services, it is mainly of interest to developers who want to inspect, audit or contribute to the code behind those web apps rather than to people looking for a standalone self-hosted product.

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.

SOGo is an open-source groupware suite that combines calendaring, address book management and a full webmail client, along with resource sharing and permission handling. It is described as fast and scalable, and it is written mainly in Objective-C with an AngularJS Material web interface. It is licensed under GPL-2.0 and developed by Alinto.

Because SOGo relies on documented standards such as IMAP, CalDAV and CardDAV, it provides native connectivity without plugins to many clients, including Microsoft Outlook, Apple iCal, the iPhone, Mozilla Lightning and many mobile devices. A Thunderbird connector extension for Thunderbird 78 and later is available in a separate repository.

Building SOGo requires the source of both SOGo and SOPE, the underlying application server framework, and a FAQ covers compilation. A dev container is available for development. The project welcomes documentation work, feature requests, mailing list participation, patches and translations, and the README lists official translations in many languages with their maintainers.