Best open-source alternatives to Microsoft Outlook

Mailu is an open-source mail server project packaged as a collection of Docker images. It is intended for people who want a full-featured email stack that is straightforward to deploy and maintain without proprietary components or unrelated groupware features. The README highlights support for standard mail protocols such as IMAP, SMTP, and Submission, along with webmail access and an administration interface. It also describes user- and admin-oriented capabilities including aliases, auto-reply, per-domain delegation, quotas, enforced TLS, DKIM, DMARC/SPF, and anti-virus/anti-spam protections.

mailcow is a dockerized mail server project aimed at people and organizations that want to self-host and manage email infrastructure. The README presents it as a packaged suite built around open-source components, with the main entry point for setup and support being the official documentation site. The project emphasizes deployment in containers and a community/support ecosystem rather than describing end-user mail features in detail on the README itself. It also notes that mailcow itself is released under GPLv3, and points readers to support contracts and a one-time SAL for paid assistance from the maintainers.

Stalwart is an open-source mail and collaboration server aimed at organizations that need a secure, scalable platform for email and groupware. It combines traditional mail protocols with collaboration features, making it suitable for teams that want a self-hosted alternative for messaging, calendaring, contacts, and file sharing. The project supports a broad set of protocols including IMAP, JMAP, POP3, SMTP, CalDAV, CardDAV, and WebDAV. It also emphasizes operational features such as multi-tenancy, authentication and authorization controls, filtering for spam and phishing, observability, high availability, and web-based administration. The README presents it as a Rust-based server designed for reliability and growth from small deployments to large clusters.

Roundcube Webmail is a self-hosted webmail client intended for users who want a browser-based email experience backed by an IMAP server. It presents itself as an application-like interface and includes common email client capabilities such as MIME handling, address book management, folder operations, search, and spell checking. The project is written primarily in PHP and JavaScript and depends on a database backend such as MariaDB, MySQL, PostgreSQL, or SQLite, plus external libraries. It is designed to run on a webserver and can be extended through a plugin API, while the user interface can be customized with skins. The README also points users to separate installation and upgrade documentation for deployment guidance.

Mail-in-a-Box is a self-hosted email appliance designed to make running a personal mail server simple. It turns a fresh Ubuntu 22.04 LTS 64-bit machine into a configured stack for receiving, sending, and managing email, with a focus on privacy, decentralization, and low-effort deployment. The project bundles the major mail-server components needed for a complete setup, including SMTP, IMAP, webmail, spam filtering, DNS, TLS certificates, backups, and monitoring. It also provides a control panel for managing users, aliases, DNS records, and backups, plus an API for control-panel actions. The README positions it for individuals who want to self-host email without dealing with a highly customizable or manually assembled server environment.

Cypht is a web-based email client and news reader designed to aggregate mail from multiple accounts into a single interface. It is aimed at users who want to manage several email sources without replacing their existing accounts, while still being able to browse folders and send messages normally. The project is built around a plugin/module-set architecture, allowing functionality to be extended or customized without modifying the core code. The README points users to installation, feature, module, security, and licensing pages, and also provides a Docker image and a Composer package for developers or deployers who want alternative installation paths.

SOGo is presented as a collaborative free and open source groupware project. The README focuses on how contributors can help improve the software through documentation, translations, feature ideas, discussion, and code patches, rather than describing end-user workflows in detail. It also explains how to obtain the source code for SOGo and its related components, including SOPE and the Thunderbird connector extension, and points readers to compilation instructions in the project FAQ. The document further highlights the software’s internationalization support by listing a broad set of official translations and maintainers, suggesting a mature, community-driven project with a strong emphasis on multilingual use and contribution.

GroupOffice is an open-source groupware and CRM platform aimed at enterprises and the public sector. It combines communication, collaboration, and business management tools into a single system, reducing the need for separate products and workflows. The project is intended for organizations that want either a self-hosted deployment or a managed cloud option from Intermesh. Its feature set includes shared files, video meetings, email, calendars, contacts, project and time tracking, quoting, invoicing, and support ticketing, and it can be extended through custom modules. The README also points developers to Docker-based production and development setups, as well as a manual source-based build process.

EGroupware is a web-based groupware system intended for teams and organizations that need shared productivity tools. The README focuses on how to install and maintain the platform rather than describing end-user features in detail, but it makes clear that the project is designed to run as a multi-container application and that a package-manager-based deployment is the preferred approach for most users. It supports multiple deployment paths, including Linux packages, Docker for non-Linux environments, and a source-based development setup. The development workflow mentions optional app repositories, installer scripts, and update channels, which suggests a modular system with separate components and maintenance routines. The project is aimed at administrators and developers who need to deploy, extend, and keep a self-hosted groupware installation up to date.

Aerion is a lightweight desktop e-mail client built as an open source project for users who want a more resource-efficient alternative to traditional mail applications. It is presented as a standalone client inspired by Geary, with an emphasis on low CPU, RAM, and battery usage, a modern interface, and keyboard-friendly navigation. The project targets Linux first, but it also supports macOS and Windows. Its feature set centers on managing multiple accounts, unified inbox handling, conversation threads, local and IMAP search, contact synchronization, and security-oriented mail capabilities such as PGP and S/MIME. It also includes a first-party extension system and custom OAuth2 support for several mail and calendar/contact protocols.

SnappyMail is a self-hosted, web-based email client built as a faster and more lightweight fork of RainLoop Webmail Community edition. It is aimed at users or administrators who want a modern browser mail interface with reduced JavaScript payloads, improved performance, and a simpler installation footprint. The project emphasizes privacy and security-oriented changes, including removal of several external service integrations, support for stronger encryption options, and updated authentication handling in the admin area. It also adds features such as dark mode, an advanced Sieve script editor, notifications via service workers, and support for Kolab groupware and richer IMAP capabilities. The README points to the project website and wiki for installation instructions and broader product information.

Email, calendar, collaboration server with Web interface and lots of integrations.