What is Citadel?
Is a collaboration suite (messaging and groupware). It is designed to run on open source operating systems such as Linux or BSD. In order to modernize the Citadel platform for the Internet, the Citadel/UX 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/UX 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.
Comments and Reviews
Said about Citadel as an alternative
citadel is the truly open source alternative