Citadel is described as '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' and is a webmail provider in the office & productivity category. There are more than 10 alternatives to Citadel for a variety of platforms, including Linux, Web-based, Windows, Self-Hosted and Mac apps. The best Citadel alternative is Kolab, which is both free and Open Source. Other great apps like Citadel are Zimbra Collaboration Suite, hMail Server, Microsoft Exchange and SmarterMail Mail Server.