Citadel is a free, open-source groupware server with email, calendars, forums, and messaging. See its rooms-based architecture, features, and history.
Citadel is a free, open-source, self-hosted groupware server that bundles email, calendaring, forums, instant messaging, wikis, blogs, and RSS aggregation into one system. Its roots trace to a 1981 bulletin board system created by Jeff Prothero, with a Unix-based version, commonly called Citadel/UX, emerging in 1987 as the direct ancestor of the software available today.
The project became fully free and open source under the GPL in 1998, and the community subsequently expanded it from a BBS platform into a general-purpose groupware server. It remains a volunteer-maintained open-source project coordinated through citadel.org rather than a commercial company.
Citadel's core concept is its rooms-based architecture, where a single navigable hierarchy of rooms can each act as an email inbox, discussion forum, calendar, file share, wiki page, or blog, unifying several communication tools under one system.
Written in C for performance and memory safety, Citadel supports standard protocols including IMAP, POP3, and SMTP alongside its WebCit web interface, and can host multiple domains from a single server instance.
Citadel is completely free under the GPL version 3 license. There is no subscription, paid tier, or vendor-managed hosting option; operators download and self-host the software on infrastructure they control and maintain themselves.
Yes. Citadel is free, open-source software distributed under the GNU General Public License, version 3. There is no paid tier or subscription.
Instead of separate apps for email, forums, and calendars, Citadel organizes everything into a navigable hierarchy of rooms, where each room can function as an inbox, forum, calendar, wiki page, or blog.
Citadel traces back to a 1981 bulletin board system by Jeff Prothero, with a Unix-based version (Citadel/UX) emerging in 1987 that is the direct ancestor of today's software. It became open source under the GPL in 1998.
Yes, a single Citadel server instance can host multiple domains, making it usable for several organizations or communities on shared infrastructure.
Yes, Citadel supports standard protocols including IMAP, POP3, and SMTP in addition to its own WebCit web interface.
No. This Citadel is unrelated to Citadel Securities or Citadel LLC, the financial firms. It is an independent open-source groupware server project hosted at citadel.org.