Open source webmail client for IMAP with address book, search, folders, and a customizable UI via plugins and skins.
7.1k stars1.8k forksGPL-3.0last commit Actively maintained
15 self-hosted Microsoft Outlook alternatives
Microsoft Outlook is an email, calendar, contacts, and tasks service with web and app clients, Exchange Online integration, mailbox and calendar management, scheduling and meeting invites, rules and filtering, and cross-device synchronization for personal and enterprise users.
Alternatives to Microsoft Outlook
#2
Lightweight CalDAV/CardDAV server for syncing calendars, to-dos, and contacts with many clients, storing data on the filesystem and supporting auth and TLS.
4.9k stars521 forksGPL-3.0last commit Actively maintained
#3
Modoboa is an open source mail server management platform with a modern web UI, integrating Postfix and Dovecot with admin tools, webmail, calendar, and address book.
3.5k stars481 forksISClast commit Actively maintained
#4
Open source groupware suite providing webmail, calendaring, address books, and shared resources via open standards like IMAP, CalDAV, and CardDAV.
2.2k stars320 forksGPL-2.0last commit Actively maintained
#5
EteSync (Etebase Server) provides a self-hostable, zero-knowledge, end-to-end encrypted sync backend for calendars, contacts, tasks and notes across devices.
1.8k stars102 forksAGPL-3.0last commit Likely dormant
SnappyMail is a fast, modern webmail client (RainLoop fork) focused on performance, privacy, and secure IMAP access with optional PGP support and admin controls.
1.7k stars215 forksAGPL-3.0last commit Actively maintained
Nextcloud Mail is a webmail app that connects to IMAP/SMTP accounts, offering a unified inbox and deep integration with other Nextcloud apps like Contacts and Calendar.
1k stars340 forksAGPL-3.0last commit Actively maintained
Open-source task scheduler and calendar app that auto-plans tasks, syncs with external calendars, and provides a modern UI for managing time blocks and workflows.
985 stars70 forksMITlast commit Actively maintained
Self-hosted Next.js React CalDAV client for managing VTODO tasks and VEVENT calendars with multi-account, list/Gantt/calendar views and filters.
625 stars42 forksGPL-3.0last commit Actively maintained
#10
Open-source PHP groupware providing CalDAV/CardDAV, ActiveSync, email client, CRM, tasks, file manager and Docker images for on-premise collaboration.
21 stars4 forksAGPL-3.0last commit Actively maintained
#11
Lightweight PHP webmail client for IMAP/SMTP servers, offering a simple interface, plugin extensibility, and broad compatibility with standard mail backends.
Activity unknown
#12
DAViCal is an open source CalDAV/WebDAV calendar server using PostgreSQL, enabling shared calendars, access delegation, and meeting scheduling across many clients.
Activity unknown
Self-hosted enterprise collaboration suite providing email, webmail, calendaring, contacts, and administration for organizations.
Activity unknown
#14
Citadel is an open source groupware and messaging server providing email, calendars, contacts, and a web interface with room-based collaboration.
Activity unknown
#15
Open-source webmail and groupware offering email, contacts, calendar, cloud drive, tasks, notes and an admin control panel for multi-tenant email services.
Activity unknown
What replacing Microsoft Outlook actually involves
Every option on this page is open source and free to run on your own hardware, so you own the data and there is no subscription to cancel. 9 of 15 shipped a commit in the last six months. Licences in this list: GPL-3.0, ISC, GPL-2.0, AGPL-3.0, MIT. In exchange you take on hosting, backups and updates yourself.
Browse everything in Calendars & Contacts (CalDAV/CardDAV).
