Zimbra is the most capable open source email and collaboration suite available today. It provides support for Email, Shared calendar, Document management, Pop3, Address book and lot more. Mails could be tagged and search support is also provided. Zimbra is considered as an alternative to Microsoft Exchange Server.

Zimbra - Email and Collaboration Suite

DavMail is a POP/IMAP/SMTP/Caldav/Carddav/LDAP exchange gateway allowing users to use any mail/calendar client (e.g. Thunderbird with Lightning or Apple iCal) with an Exchange server, even from the internet or behind a firewall through Outlook Web Access. DavMail now includes an LDAP gateway to Exchange global address book and user personal contacts to allow recipient address completion in mail compose window and full calendar support with attendees free/busy display. 
The main goal of DavMail is to provide standard compliant protocols in front of proprietary Exchange. This means LDAP for global address book, SMTP to send messages, IMAP to browse messages on the server in any folder, POP to retrieve inbox messages only, Caldav for calendar support and Carddav for personal contacts sync. Thus any standard compliant client can be used with Microsoft Exchange.

DavMail is a POP/IMAP/SMTP/Caldav/Carddav/LDAP exchange gateway allowing users to use any mail/calendar client (e.g. Thunderbird with Lightning or Apple iCal) with an Exchange server, even from the internet or behind a firewall through Outlook Web Access. DavMail now includes an LDAP gateway to Exchange global address book and user personal contacts to allow recipient address completion in mail compose window and full calendar support with attendees free/busy display. 

DavMail - POP/IMAP/SMTP/Caldav to Exchange

James (a.k.a Java Apache Mail Enterprise Server) is a 100% pure Java SMTP and POP3 Mail server, and NNTP News server designed to be a complete and portable enterprise mail/messaging engine solution based on currently available open messaging protocols. 
 <ul><li>Is 100% pure java capable of running on Java 1.4 onwards</li>
<li>Integrates protocols
 <ul><li>Email
 <ul><li><a href="http://james.apache.org/server/rfclist/smtp/rfc0821.txt" class="externalLink"><abbr title="Simple Mail Transport Protocol">SMTP</abbr></a></li>
<li><a href="http://james.apache.org/server/rfclist/pop3/rfc1725.txt" class="externalLink"><abbr title="Post Office Protocol">POP3</abbr></a><ul><li>Outgoing server</li>
<li>Incoming from POP3 accounts (FetchMail)</li>
</ul>
</li>
<li><a href="http://james.apache.org/server/rfclist/imap4/rfc2060.txt" class="externalLink"><abbr title="Internet Message Access Protocol">IMAP</abbr></a> (James 3)
 <ul><li>Server</li>
<li><a href="http://james.apache.org/jsieve" class="externalLink">Sieve</a> 
 filtering into IMAP mailboxes for incoming mail</li>
</ul>
</li>
</ul>
</li>
</ul>
<ul><li><a href="http://james.apache.org/server/rfclist/nntp/rfc0977.txt" class="externalLink"><abbr title="Network News Transfer Protocol">NNTP</abbr></a> (better known as news)</li>
</ul>
</li>
<li>Is based on open standards</li>
<li>Is a mailet container. Processing is delegated to independent, extensible, pluggable agents
 specified by the <a href="http://james.apache.org/mailet/api" class="externalLink">Mailet API</a>. Any function
 which is not already available (from <a href="http://james.apache.org/mailet" class="externalLink">James</a>
 or from a third party) can be developed.</li>
<li>Is a modular, component based <a href="http://dbpedia.org/resource/Inversion_of_control" class="externalLink">IoC</a> 
 mail platform
 <ul><li>Based on mature, production proved <a href="http://excalibur.apache.org" class="externalLink">Apache Excalibur</a>
 components</li>
<li>James 2 (production) uses the <a href="http://avalon.apache.org" class="externalLink">Avalon</a> framework. Avalon development
 has now stopped but the framework is mature, stable and of proved production quality.</li>
<li><a href="http://james.apache.org/server/head" class="externalLink">James 3</a> (development) supports Spring and is moving
 towards OSGI</li>
</ul>
</li>
<li>Support to create mailing list </li>

</ul>

James (a.k.a Java Apache Mail Enterprise Server) is a 100% pure Java SMTP and POP3 Mail server, and NNTP News server designed to be a complete and portable enterprise mail/messaging engine solution based on currently available open messaging protocols.

James - Enterprise Mail Server

Mailu is a simple yet full-featured mail server as a set of Docker images. The project aims at providing people with an easily setup, easily maintained and full-featured mail server while not shipping proprietary software nor unrelated features often found in popular groupware.
 
Its features include: 
<ul>
<li>Standard email server, IMAP and IMAP+, SMTP and Submission</li>
<li>Advanced email features, aliases, domain aliases, custom routing</li>
<li>Web access, multiple Webmails and administration interface</li>
<li>User features, aliases, auto-reply, auto-forward, fetched accounts</li>
<li>Admin features, global admins, announcements, per-domain delegation, quotas</li>
<li>Security, enforced TLS, Letsencrypt!, outgoing DKIM, anti-virus scanner</li>
<li>Antispam, auto-learn, greylisting, DMARC and SPF</li>
<li>Freedom, all FOSS components, no tracker included</li>
</ul>

Mailu is a simple yet full-featured mail server as a set of Docker images. The project aims at providing people with an easily setup, easily maintained and full-featured mail server while not shipping proprietary software nor unrelated features often found in popular groupware.

Mailu - Mail server as a set of Docker images

iRedMail is a fully fledged, full-featured mail server solution. It uses open source packages like Postfix, Dovecot, Apache, MySQL, OpenLDAP, Amavisd, RoundCube etc to provide this solution. 

IRedMail - Free, Open Source Email Server Solution for Linux

hMailServer is an open source email server for Microsoft Windows. It supports the common e-mail protocols (IMAP, SMTP and POP3) and can easily be integrated with many existing web mail systems. It has flexible score-based spam protection and can attach to your virus scanner to scan all incoming and outgoing email. 

hMailServer - E-Mail Server for Microsoft Windows. 

WildDuck is a modern mail server software for IMAP and POP3. Modern being scalable, unicode-first and API-controlled. You can bundle WildDuck with Haraka and ZoneMTA to create a complete mail server. It uses a distributed database (sharded + replicated MongoDB) as a backend for storing all data, including emails. It tries to follow Gmail in product design. If there's a decision to be made then usually the answer is to do whatever Gmail has done.

WildDuck Mail Server -  Self hosted modern mail server `

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Projects

Zimbra - Email and Collaboration Suite

DavMail - POP/IMAP/SMTP/Caldav to Exchange

James - Enterprise Mail Server

Mailu - Mail server as a set of Docker images

IRedMail - Free, Open Source Email Server Solution for Linux

hMailServer - E-Mail Server for Microsoft Windows.

WildDuck Mail Server - Self hosted modern mail server `

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Zimbra - Email and Collaboration Suite

DavMail - POP/IMAP/SMTP/Caldav to Exchange

James - Enterprise Mail Server

Mailu - Mail server as a set of Docker images

IRedMail - Free, Open Source Email Server Solution for Linux

hMailServer - E-Mail Server for Microsoft Windows.

WildDuck Mail Server - Self hosted modern mail server `

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License