Exchange Replacement: Sogo/Openchange / Zimbra Zentyal Exchange Open source alternatives

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EGroupware is free open-source groupware software intended for businesses from small to enterprises. Its primary functions allow users to manage contacts, appointments, projects and to-do lists.

SOGo (Opensource) Share your calendars, address books and mails in your community with a completely free and open source solution.

Zimbra (Opensource) Zimbra Collaboration Open Source Edition provides administrators and their end users innovative features that solve the challenges faced by organizations using today's legacy communications infrastructure.

Zentyal
Zentyal is an open source email and groupware solution based on Ubuntu Linux. Zentyal implements the Microsoft Exchange Server protocols on top of standard open source components in order to provide native compatibility with Microsoft Outlook clients.


Citadel (Opensource) Easy, versatile, and powerful, thanks to its exclusive "rooms" based architecture.

iRedMail (Opensource) A zero cost, fully fledged, full-featured mail server solution.

Kolab (Opensource) The world's most loved open source collaboration software.

hMailServer (Opensource) Opensource mail server for Microsoft Windows.

MTA
Client
N1 (Opensource) An extensible mail client built on the modern web.

Webclient
AfterLogic WebMail Lite (Opensource, Demo) Fast and easy-to-use webmail front-end for your existing IMAP mail server, Plesk or cPanel.

Cypht (Opensource) All your E-mail, from all your accounts, in one place. Cypht is like a news reader, but for E-mail. Cypht does not replace your existing accounts - it combines them into one. And it's also a news reader :).

Mailpile (Opensource) Mailpile is an effort to reclaim private communication on the Internet. A project to rescue our personal lives from the proprietary cloud and prevent our conversations from being strip-mined for corporate profit and government surveillance.

ProtonMail WebClient (Opensource) Official AngularJS Web Client for ProtonMail.

RainLoop (Opensource, Demo) Simple, modern & fast web-based email client.

Roundcube (Opensource, Demo) A browser-based multilingual IMAP client with an application-like user interface.

SquirrelMail (Opensource, Demo) Standards-based webmail package written in PHP. It includes built-in pure PHP support for the IMAP and SMTP protocols, and all pages render in pure HTML 4.0 (with no JavaScript required) for maximum compatibility across browsers.

All-in-one
Axigen (Free, Demo) The Axigen Free Mail Server is a great alternative to open source. It is a turnkey messaging solution, perfect for small & micro businesses, integration projects or test environments.

Mailcow (Opensource) Mailserver suite based on Dovecot, Postfix and other open source software, that provides a modern Web UI for administration. In future versions mailcow will provide Cal- and CardDAV support.

Mail-in-a-Box (Opensource) Mail-in-a-Box lets you become your own mail service provider in a few easy steps. It’s sort of like making your own gmail, but one you control from top to bottom.

Modoboa (Opensource, Demo) Mail hosting and management platform including a modern and simplified Web User Interface. It provides useful components such as an administration panel or a webmail.

PEPS (Opensource) Open source Email + File sharing with end-to-end encryption.

Poste (Free, Demo) Complete mail server built in docker.

Gateway
CipherMail (Free) Centrally managed email encryption at the gateway level and on mobile devices.

Scrollout F1 (Free) An easy to use, already adjusted email gateway (firewall) offering free anti-spam, anti-virus protection in order to secure all existing email servers, such as Microsoft Exchange, Lotus Domino, Postfix, Exim, Qmail and more.

MailCleaner (Free) MailCleaner is a full email filtering gateway. It includes a complete GNU/Linux OS and a graphical web interface for user and administrative access.

Xeams (Free) Free email server that can be used by any corporation to handle their enterprise email. It works on any operating system and can handle unlimited number of emails and domains. Xeams can work in 3 modes, Stand alone, Spam firewall or Hybrid mode.

Tools
ViMbAdmin (Opensource) The ViMbAdmin provides a web based virtual mailbox administration system allowing mail administrators to manage domains, mailboxes and aliases.

Openchange
Openchange is the connector that is used by all Ms Exchange compatible servers.

Jmap
Jmap is a way to increase the speed of the imap protocol to be more like gmail. It is currently used by Fastmail.

Gitter.im
Gitter is a public chat service used by developers