Citadel is a turnkey open-source solution for email and collaboration. One simple installation delivers a multitude of powerful features, including:

email calendaring/scheduling address books bulletin boards mailing list server instant messaging multiple domain support modern AJAX-style web interface

Users love Citadel because it's software that helps them work together and stay in touch without calling attention to itself. System administrators love Citadel because it installs in minutes without the need to manually integrate all the different components together.The key to Citadel's versatility is its unique architecture. A Citadel system is made up of containers called "rooms." A room may be used as an email folder, a discussion forum, a real-time chat, a mailing list, a calendar, an address book, an RSS reader … sometimes a combination of any of the above, and certainly any other uses which could be added in the future. You can also distribute a Citadel installation across multiple servers, allowing unlimited horizontal scalability.

With that in mind, Citadel excels at applications such as:

• E-mail and groupware (shared calendaring, etc.)
• Bulletin Board System (BBS)
• Online discussion forums / chat rooms

The Citadel system is extremely versatile. It provides numerous front ends to present to users, such as a text-based interface, an AJAX-style web interface, and many popular PIM clients using SMTP/POP/IMAP. All of these can be used simultaneously.It's also extremely scalable. Not only can a well-equipped Citadel server support a large number of concurrent users, but you can also build a distributed network of Citadel nodes that share rooms and their content.

• Email, calendaring, address books, bulletin boards, instant messaging, and more … all in one tightly integrated server package. Unlike other open source groupware systems, all of Citadel's data stores are built-in. “Integrate-it-yourself” is a thing of the past.
• High-performance, multiprotocol, multithreaded server engine
• Web browser, telnet/SSH, local client software accessible
• Standards-compliant e-mail built in: IMAP, POP3, ESMTP
• Group calendaring and scheduling (GroupDAV and Kolab-1 compatible)
• Built-in listserv (mailing list server)
• 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.
• Database-driven, single-instance message store
• Built-in full text index for fast searching
• Authenticated SMTP (no more tedious mucking about with POP-before-SMTP hacks!)
• Multiple domain support
• Built-in integration with perimiter email filtering technologies such as Realtime Blackhole Lists (RBL's) and SpamAssassin
• Very strong replication features. 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.
• Web-based access to email, calendars, and everything else through a powerful AJAX-style front end
• Very strong support for “public folders” and message forums.
• Built-in instant messenger service
• SSL/TLS encryption for all protocols
• LDAP integration capabilities. Citadel can populate your existing directory server, or it can configure a standalone directory server if you don't already have one.
• Citadel is here today and is actively maintained – it's neither vaporware nor abandonware.
• Citadel is true open source (GPL) software. Unlike other groupware servers, it isn't a cut-down version of an expensive proprietary “pro” version. We make our very best work available to everyone on the same terms.