Citadel/UX

{{Infobox software

| name = Citadel/UX

| logo = Citadel logo-final-o.svg

| logo alt =

| logo caption =

| screenshot = Screenshot of citadel groupware 2011.png

| screenshot_size =

| caption = Screenshot of citadel groupware 2011

| released = Before {{Start date and age|1988}}

| latest release version = {{wikidata|property|preferred|references|edit|Q1093291|P348|P548=Q2804309}}

| latest release date = {{wikidata|qualifier|preferred|single|Q1093291|P348|P548=Q2804309|P577}}

| latest preview version = {{wikidata|property|preferred|references|edit|Q1093291|P348|P548=Q51930650}}

| latest preview date = {{wikidata|qualifier|preferred|single|Q1093291|P348|P548=Q51930650|P577}}

| developer = {{URL|citadel.org}}

| operating_system = Unix, Linux

| genre = Groupware

| programming language = C

| license = 2007: GPL-3.0-only{{cite web|title=Updated the copyright declaration in several modules|url=https://code.citadel.org/?p=citadel.git;a=commit;h=aa7365c86de8e26e796d3aa3fd605c85d8c26220|date=2012-07-02}}{{cite web|title=More license declarations|url=https://code.citadel.org/?p=citadel.git;a=commit;h=8c47559cb5ae97ec0fa35660ee16fd61a9451c72|date=2009-10-15}}{{cite web|title=Began (but did not finish) applying GPL3+ declarations to each source file.|url=https://code.citadel.org/?p=citadel.git;a=commit;h=4eb74b26380dfde31c86c685f0589e0c653aebf0|date=2009-10-14}}{{cite web|title=Move to GPL v3|url=https://code.citadel.org/?p=citadel.git;a=commit;h=b2971ed49d2cfe5128190ec1d5d8c0fa239f0004|date=2007-07-02}}
2006: GPL-2.0-only{{cite web|title=Clarify that we are GPLv2|url=https://code.citadel.org/?p=citadel.git;a=commit;h=b7d79f725dac5c4853a8a1ddf64c28a035388c12|date=2006-01-26}}
1998: GPL-2.0-or-later{{cite web|title=Citadel/UX release 5.01|url=https://code.citadel.org/?p=citadel.git;a=blob;f=citadel/copyright.txt;h=3f08f6b255a11f67bbe28e77cacf8edea4175034;hb=54b54a07b29cd57a19728f468e19cf887cda3873|date=1998-07-11}}

| website = {{URL|https://citadel.org/}}

}}

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.{{Cite web |last=Emms |first=Steve |date=2023-10-31 |title=Citadel - groupware software |url=https://www.linuxlinks.com/citadel/ |access-date=2025-02-22 |website=LinuxLinks |language=en-GB}}

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.{{Cn|date=March 2025}}

Citadel uses the Berkeley DB database for all of its data stores, including the message base.{{Cn|date=March 2025}}

Citadel became free and open-source software subject to the terms of the GPL-2.0-or-later license in 1998.{{Cite web|title=[seul-edu] Citadel/UX|url=https://archives.seul.org/seul/edu/Dec-1999/msg00203.html|access-date=2021-08-05|website=archives.seul.org}} In 2006 Citadel was relicensed to the GPL-2.0-only license. In 2007 Citadel was relicensed again to the GPL-3.0-only license.{{cite web|title=Citadel adopts GPLv3|url=http://www.citadel.org/doku.php/news:citadel.adopts.gplv3|quote=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.|date=2007-07-30|archive-url=https://web.archive.org/web/20070817101740/http://www.citadel.org/doku.php/news:citadel.adopts.gplv3|archive-date=2007-08-17}}