20 self-hosted Microsoft Exchange Online alternatives

Nextcloud is an open-source content collaboration platform providing file sync & share, groupware (calendar, contacts, mail), real-time office editing, chat and video conferencing.

36.3k stars5.1k forksAGPL-3.0last commit 28 Jul 2026Actively maintained

Production-ready mail server stack in a Docker container with SMTP, IMAP/POP3, LDAP auth, anti-spam/AV, DKIM/DMARC, and optional OAuth2 support.

18.6k stars2k forksMITlast commit 18 Jul 2026Actively maintained

Mail-in-a-Box is a one-click mail server appliance for Ubuntu, bundling SMTP/IMAP, webmail, DNS, TLS automation, backups, and an admin control panel.

15.4k stars1.5k forksCC0-1.0last commit 24 May 2026Actively maintained

All-in-one open-source mail and collaboration server with SMTP, IMAP, JMAP, CalDAV, CardDAV, and WebDAV, plus integrated anti-spam and phishing protection.

13.9k stars848 forkslast commit 27 Jul 2026Actively maintained

Mailcow is a dockerized mail server suite providing SMTP/IMAP, webmail, anti-spam/anti-virus, and domain/mailbox administration via a unified web UI.

13.2k stars1.8k forksGPL-3.0last commit 15 Jul 2026Actively maintained

Mailu is a full-featured mail server distributed as Docker images, providing SMTP/IMAP/POP3, webmail, administration UI, and built-in security and anti-spam features.

7.4k stars990 forkslast commit 26 Jul 2026Actively maintained

Go-based, modular mail server implementing SMTP/LMTP/IMAP with built-in DKIM/SPF/DMARC, storage backends (SQLite/Postgres/S3), Prometheus metrics and Docker support.

6k stars324 forksGPL-3.0last commit 13 Jul 2026Actively maintained

All-in-one secure mail server with SMTP, IMAP, webmail, automatic TLS (ACME), and built-in SPF/DKIM/DMARC plus junk filtering for low-maintenance domains.

5.8k stars210 forksMITlast commit 20 Jul 2026Actively maintained

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 stars515 forksGPL-3.0last commit 19 Jul 2026Actively maintained

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 stars476 forksISClast commit 27 Jul 2026Actively maintained

Open-source Debian-based OS and control panel that simplifies installing, managing and securing web apps, mail and services via a web admin and app catalog.

3k stars349 forksAGPL-3.0last commit 30 Jun 2026Actively maintained

A Bash script that auto-implements a full mail server (Postfix, Dovecot, DKIM, SpamAssassin) with TLS and PAM-based logins on Debian/Ubuntu.

2.2k stars340 forksGPL-3.0last commit 4 May 2025Slowing down

Open source groupware suite providing webmail, calendaring, address books, and shared resources via open standards like IMAP, CalDAV, and CardDAV.

2.1k stars311 forksGPL-2.0last commit 15 Jul 2026Actively maintained

WildDuck is a horizontally scalable IMAP/POP3 mail server that stores all mail in MongoDB, runs stateless Node.js workers, and provides a REST API for full server control.

2.1k stars276 forksEUPL-1.2last commit 27 Jul 2026Actively maintained

Automated open-source mail server installer bundling Postfix, Dovecot, webmail, anti-spam/antivirus and LDAP/SQL backends for mainstream Linux/BSD.

1.8k stars259 forksGPL-3.0last commit 22 Jul 2026Actively maintained

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 stars101 forksAGPL-3.0last commit 12 Jul 2024Likely dormant

sabre/dav is a PHP framework for building WebDAV file servers and CalDAV/CardDAV calendar and contacts services.

1.7k stars371 forksBSD-3-Clauselast commit 15 Jul 2026Actively maintained

High-performance, standards-compliant IMAP/POP3/LMTP server with flexible authentication, extensible plugin system (Lua, Sieve) and OpenSSL TLS support.

1.2k stars345 forkslast commit 24 Jul 2026Actively maintained

Davis is a sabre/dav-based CalDAV/CardDAV/WebDAV server with a modern Symfony 7 admin dashboard for managing users, calendars, sharing, and address books.

726 stars45 forksMITlast commit 16 Jun 2026Actively maintained

Cyrus IMAP is a secure, scalable mail server providing IMAP/JMAP email plus CalDAV/CardDAV contacts and calendars, with ACLs, quotas, search, and replication.

637 stars173 forkslast commit 27 Jul 2026Actively maintained