Citadel is a collaboration suite (messaging and groupware). It provides support for Email, Calendaring/Scheduling,  
  Address books, Bulletin boards, Mailing List Server, Instant Messaging, Wiki, Multiple domain support. <BR><BR>Its features include: <BR>
  <ul>
<li class="level1"><div class="li"> Email, calendaring, address books, bulletin boards, instant messaging, and more &hellip; all in <strong>one</strong> tightly integrated server package. Unlike other open source groupware systems, <strong>all of Citadel&#039;s data stores are built-in.</strong> All that tedious mucking about with dependencies and config files is a thing of the past.   </div>
</li>
<li class="level1"><div class="li"> High-performance, multiprotocol, multithreaded server engine   </div>
</li>
<li class="level1"><div class="li"> Web browser, telnet/<acronym title="Secure Shell">SSH</acronym>, local client software accessible   </div>
</li>
<li class="level1"><div class="li"> Standards-compliant e-mail built in: <acronym title="Internet Message Access Protocol">IMAP</acronym>, <acronym title="Post Office Protocol 3">POP3</acronym>, <acronym title="Extended Simple Mail Transfer Protocol">ESMTP</acronym>   </div>
</li>
<li class="level1"><div class="li"> Group calendaring and scheduling (<a href="http://www.groupdav.org" class="urlextern" title="http://www.groupdav.org">GroupDAV</a> and Kolab-1 compatible)   </div>
</li>
<li class="level1"><div class="li"> Built-in listserv (mailing list server)</div>
</li>
<li class="level1"><div class="li"> Integrated server-side mail sorting and filtering.  Users can choose between an easy-to-use web based rules editor, or the power of writing complex scripts using the industry standard Sieve language.</div>
</li>
<li class="level1"><div class="li"> <a href="http://bionicmessage.net/index.php?q=node/11" class="urlextern" title="http://bionicmessage.net/index.php?q=node/11">Support for push e-mail and mobile devices</a></div>
</li>
<li class="level1"><div class="li"> Database-driven, single-instance message store</div>
</li>
<li class="level1"><div class="li"> Built-in full text index for fast searching</div>
</li>
<li class="level1"><div class="li"> Authenticated <acronym title="Simple Mail Transfer Protocol">SMTP</acronym> for remote email submission</div>
</li>
<li class="level1"><div class="li"> Multiple domain support   </div>
</li>
<li class="level1"><div class="li"> Built-in integration with perimiter email filtering technologies such as Realtime Blackhole Lists (RBL&#039;s), <a href="http://spamassassin.apache.org/" class="urlextern" title="http://spamassassin.apache.org/">SpamAssassin</a>, and <a href="http://www.clamav.net" class="urlextern" title="http://www.clamav.net">ClamAV antivirus</a></div>
</li>
<li class="level1"><div class="li"> Server-to-server replication. Users in any number of domains can be spread out across any number of Citadel servers, allowing you to put data where you need it, and enabling infinite horizontal scalability.   </div>
</li>
<li class="level1"><div class="li"> Web-based access to email, calendars, and everything else through a powerful <acronym title="Asynchronous JavaScript and XML">AJAX</acronym>-style front end   </div>
</li>
<li class="level1"><div class="li"> Very strong support for “public folders” and message forums.   </div>
</li>
<li class="level1"><div class="li"> Built-in instant messenger service   </div>
</li>
<li class="level1"><div class="li"> <acronym title="Secure Sockets Layer">SSL</acronym>/TLS encryption for all protocols   </div>
</li>
<li class="level1"><div class="li"> Citadel is <em>true open source</em> software.  Unlike other groupware servers, it isn&#039;t a cut-down version of an expensive proprietary “pro” version.  We make our very best work available to everyone on the same terms.  100% of our code is covered by the GNU General Public License (GPL3).</div>
</li>
</ul>

Citadel is a collaboration suite (messaging and groupware). It provides support for Email, Calendaring/Scheduling, Address books, Bulletin boards, Mailing List Server, Instant Messaging, Wiki, Multiple domain support.

Citadel

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

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.<BR><BR>
 <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.
<BR><BR>
Its features include: <BR>
<ul>
<li><p><strong>Standard email server</strong>, IMAP and IMAP+, SMTP and Submission</p></li>
<li><p><strong>Advanced email features</strong>, aliases, domain aliases, custom routing</p></li>
<li><p><strong>Web access</strong>, multiple Webmails and administration interface</p></li>
<li><p><strong>User features</strong>, aliases, auto-reply, auto-forward, fetched accounts</p></li>
<li><p><strong>Admin features</strong>, global admins, announcements, per-domain delegation, quotas</p></li>
<li><p><strong>Security</strong>, enforced TLS, Letsencrypt!, outgoing DKIM, anti-virus scanner</p></li>
<li><p><strong>Antispam</strong>, auto-learn, greylisting, DMARC and SPF</p></li>
<li><p><strong>Freedom</strong>, all FOSS components, no tracker included</p></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

Postfix is mail server that started life at IBM research as an alternative to the widely-used Sendmail program. Postfix attempts to be fast, easy to administer, and secure. The outside has a definite Sendmail-ish flavor, but the inside is completely different. It supports IPv6, DKIM, DomainKeys and SenderID authentication, SPF, Sendmail Milter (mail filter) protocol, Zombie blocker, TLS encryption, LDAP and it supports database like MySQL, PostgreSQL, Berkeley DB, Memcache.

Postfix - Mail Server

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

The deepOfix Mail Server makes it possible for organisations with minimal technical expertise to have robust messaging infrastructure. It has built in LDAP support, SMTP, POP3 and IMAP services.

deepOfix - Mail Server

JES is a multi-featured hybrid MTA/MDA server written in the java programming language. It supports TLS, SSL, SMTP authentication, spam control and lot more. 

Java Email Server

C# Email Server (CES). Implements SMTP and POP3 components that can be used as a standalone server or as components in other applications. It is a port of the Java Email Server (JES). This component allows a program to accept incoming emails and process them using user defined logic.

C# Email Server - SMTP and POP3 Email Server in CSharp

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 `

CRSMail a 100% JAVA based E-Mail server. It supports SMTP, POP3, Plugins, Can listen on multiple ports, Log's everything to a log file etc.

CRSMail - JAVA based E-Mail server

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