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

Claws Mail is a GTK+ based, lightweight, and fast e-mail client and newsreader. Supports POP3, APOP, IMAP, SMTP, SMTP AUTH, NNTP, LDAP, multiple accounts, spell-checking, addressbook, SSL, GPG, filtering, i18n, & more. It is further extended by plugins. Lots of extra functionality, like an RSS aggregator, calendar, or laptop LED handling, are provided by extra plugins. 

Claws Mail - Email Client and News Reader Based on GTK+

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.

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Exim - Message Transfer Agent

Dovecot is an open source IMAP and POP3 email server for Linux or UNIX-like systems written with security primarily in mind. Dovecot is an excellent choice for both small and large installations. It is fast, simple to set up, requires no special administration and it uses very little memory. Dovecot is NOT responsible for receiving mail from other servers. Dovecot presents mail already stored on the system to mail-user agents [MUA].

Dovecot - Secure IMAP and Pop3 server 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

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

Fetchmail is a full-featured, robust, well-documented remote-mail retrieval and forwarding utility intended to be used over on-demand TCP/IP links (such as SLIP or PPP connections). It supports every remote-mail protocol now in use on the Internet: POP2, POP3, RPOP, APOP, KPOP, all flavors of IMAP, ETRN, and ODMR. It can even support IPv6 and IPSEC. 
<BR><BR>
Fetchmail retrieves mail from remote mail servers and forwards it via SMTP, so it can then be read by normal mail user agents such as mutt, elm(1) or BSD Mail. It allows all your system MTA's filtering, forwarding, and aliasing facilities to work just as they would on normal mail. Fetchmail can be used as a POP/IMAP-to-SMTP gateway for an entire DNS domain, collecting mail from a single drop box on an ISP and SMTP-forwarding it based on header addresses. (We don't really recommend this, though, as it may lose important envelope-header information. ETRN or a UUCP connection is better.)

Fetchmail is a full-featured, robust, well-documented remote-mail retrieval and forwarding utility intended to be used over on-demand TCP/IP links (such as SLIP or PPP connections). It supports every remote-mail protocol now in use on the Internet: POP2, POP3, RPOP, APOP, KPOP, all flavors of IMAP, ETRN, and ODMR. It can even support IPv6 and IPSEC.

fetchmail - TLS-enabled mail retrieval and forwarding agent

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