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

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

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

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

OpenChange is a portable Open Source implementation of Microsoft Exchange server and Exchange protocols. It provides a complete solution to interoperate with Microsoft Outlook clients or Microsoft Exchange servers. <br><br>
OpenChange client-side library is used in existing messaging clients and is the solution in new projects to communicate natively with Microsoft Exchange and Exchange-compatible servers. OpenChange server is a transparent Microsoft Exchange server replacement using native Exchange protocols and does not require any plugin installation in Outlook.

OpenChange is a portable Open Source implementation of Microsoft Exchange server and Exchange protocols. It provides a complete solution to interoperate with Microsoft Outlook clients or Microsoft Exchange servers.


OpenChange - Open Source implementation of Microsoft Exchange server

Hermes Secure Email Gateway is a Free Open Source Email Gateway that provides Spam, Virus and Malware protection, full in-transit and at-rest email encryption as well as email archiving. 
<BR><BR>
Hermes Secure Email Gateway combines Open Source technologies such as Postfix, Apache SpamAssassin, ClamAV, Amavisd-new and CipherMail under one unified web based Web GUI for easy administration and management of your incoming and ougoing email for your organization.
<BR><BR>
It can be deployed to protect your in-house email solution as well as cloud email solutions such as Google Mail and Microsoft Office 365.

Hermes Secure Email Gateway is a Free Open Source Email Gateway that provides Spam, Virus and Malware protection, full in-transit and at-rest email encryption as well as email archiving. 

Hermes Secure Email Gateway

DBMail is a collection of programs that enables email to be stored in and retrieved from a database. It is scalable. You can run the dbmail programs on different servers talking to the same database(cluster). LDAP integration allows you to integrate with existing authentication backends, such as ActiveDirectory.

DBMail - Fast and scalable sql based email services

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