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. <BR><BR>
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.<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

Zentyal is a Linux Small Business Server, good alternative Windows Business Server. Zentyal can act as a Gateway, Infrastructure Manager, Unified Threat Manager, Office Server, Unified Communication Server or a combination of them. It is built on Ubuntu Linux distribution. This package includes LDAP server, Mail server, Firewall, Network infrastructure, VPN support, Web server, File server, Print server, FTP server, Groupware, VoIP server, Virtual Machines management and lot more.

Zentyal - Linux Small Business Server

 Haraka is a highly scalable node.js email server with a modular plugin architecture. Haraka can serve thousands of concurrent connections and deliver thousands of messages per second. Haraka and plugins are written in asynchronous JS and are very fast. Haraka has very good spam protection (see plugins) and works well as a filtering MTA. It also works well as a MSA running on port 587 with auth and dkim_sign plugins enabled. 
<BR><BR>
Haraka has a scalable outbound mail delivery engine built in. Mail marked as relaying (such as via an auth plugin) is automatically queued for outbound delivery.

Haraka is a highly scalable node.js email server with a modular plugin architecture. Haraka can serve thousands of concurrent connections and deliver thousands of messages per second. Haraka and plugins are written in asynchronous JS and are very fast. Haraka has very good spam protection (see plugins) and works well as a filtering MTA. It also works well as a MSA running on port 587 with auth and dkim_sign plugins enabled.

Haraka - A fast, highly extensible, and event driven SMTP 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

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

Maddy Mail Server implements all functionality required to run a e-mail server. It can send messages via SMTP (works as MTA), accept messages via SMTP (works as MX) and store messages while providing access to them via IMAP. In addition to that it implements auxiliary protocols that are mandatory to keep email reasonably secure (DKIM, SPF, DMARC, DANE, MTA-STS). It replaces Postfix, Dovecot, OpenDKIM, OpenSPF, OpenDMARC and more with one daemon with uniform configuration and minimal maintenance cost.

Maddy - Composable all-in-one mail server

Postal is a complete and fully featured mail server for use by websites & web servers. Think Sendgrid, Mailgun or Postmark but open source and ready for you to run on your own servers. Postal was developed by aTech Media to serve its own mail processing requirements and we have since decided that it should be released as an open source project for the community.
<BR><BR>
The application has been running in production for us for nearly 6 months and we will be continuing to use it ourselves and support its ongoing development. If you have any questions about getting up and running, just post an issue.

Postal is a complete and fully featured mail server for use by websites & web servers. Think Sendgrid, Mailgun or Postmark but open source and ready for you to run on your own servers. Postal was developed by aTech Media to serve its own mail processing requirements and we have since decided that it should be released as an open source project for the community.

Postal - A fully featured open source mail delivery platform for incoming & outgoing e-mail

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

MailCatcher runs a super simple SMTP server which catches any message sent to it to display in a web interface. Run mailcatcher, set your favourite app to deliver to smtp://127.0.0.1:1025 instead of your default SMTP server, then check out http://127.0.0.1:1080 to see the mail that's arrived so far.

MailCatcher - Catches mail and serves it through a dream

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

Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet. There is a great deal of flexibility in the way mail can be routed, and there are extensive facilities for checking incoming mail.

Exim - Message Transfer Agent

Modoboa is a 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. Modoboa integrates with well known software such as Postfix or Dovecot. A SQL database (MySQL, PostgreSQL or SQLite) is used as a central point of communication between all components.

Modoboa - Mail hosting made simple

Mail-in-a-Box helps individuals take back control of their email by defining a one-click, easy-to-deploy SMTP+everything else server: a mail server in a box. 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. Technically, Mail-in-a-Box turns a fresh cloud computer into a working mail server. But you don’t need to be a technology expert to set it up.

Mail-in-a-Box - One click easy-to-deploy SMTP server

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