- 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.

#Discussion (interactive)
irc://irc.freenode.net/citadel
#Discussion (forum)
https://uncensored.citadel.org