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Initial releaseBefore 1988; 36 years ago (1988)
Stable release
996[1] Edit this on Wikidata / 29 September 2023
Written inC
Operating systemUnix, Linux
License2007: GPL-3.0-only[2][3][4][5]
2006: GPL-2.0-only[6]
1998: GPL-2.0-or-later[7]

Citadel (originally referred to as "Citadel/UX" to disambiguate it from other implementations) is a collaboration suite (messaging and groupware) that is directly descended from the Citadel family of programs which became popular in the 1980s and 1990s as a bulletin board system platform. It is designed to run on open source operating systems such as Linux or BSD. Although it is being used for many bulletin board systems, in 1998 the developers began to expand its functionality to a general purpose groupware platform.

In order to modernize the Citadel platform for the Internet, the Citadel developers added functionality such as shared calendars, instant messaging, and built-in implementations of Internet protocols such as SMTP, IMAP, Sieve, POP3, GroupDAV and XMPP. All protocols offer OpenSSL encryption for additional security.

Users of Citadel systems also have available to them a web-based user interface which employs Ajax style functionality to allow application-like interaction with the system.

Citadel uses the Berkeley DB database for all of its data stores, including the message base.

Citadel became free and open-source software subject to the terms of the GPL-2.0-or-later license in 1998.[7][8] In 2006 Citadel was relicensed to the GPL-2.0-only license.[6] In 2007 Citadel was relicensed again to the GPL-3.0-only license.[5][9]


  1. ^ "Release version 996".
  2. ^ "Updated the copyright declaration in several modules". 2012-07-02.
  3. ^ "More license declarations". 2009-10-15.
  4. ^ "Began (but did not finish) applying GPL3+ declarations to each source file". 2009-10-14.
  5. ^ a b "Move to GPL v3". 2007-07-02.
  6. ^ a b "Clarify that we are GPLv2". 2006-01-26.
  7. ^ a b "Citadel/UX release 5.01". 1998-07-11.
  8. ^ "[seul-edu] Citadel/UX". archives.seul.org. Retrieved 2021-08-05.
  9. ^ "Citadel adopts GPLv3". 2007-07-30. Archived from the original on 2007-08-17. The developers of the Citadel messaging and collaboration system are pleased to announce that we are now releasing all of our software under version 3 of the GNU General Public License.

External links[edit]