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' and is an app in the Office & Productivity category. There are more than 10 alternatives to Citadel for a variety of platforms, including Online / Web-based, Windows, Linux, Self-Hosted solutions and Mac. The best alternative is Zimbra Collaboration Suite, which is both free and Open Source. Other great apps like Citadel are hMail Server (Free, Open Source), Kolab Community (Freemium, Open Source), Microsoft Exchange Server (Paid) and MDaemon Messaging Server (Paid).
Zimbra Collaboration is an open, secure collaboration tool for email, calendaring, tasks and more. With the most innovative web application available today, Zimbra boosts the productivity of users on any desktop and dramatically reduces TCO compared to legacy platform vendors.
Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft. It is part of the Microsoft Servers line of server products and is widely used by enterprises using Microsoft infrastructure solutions.
Low cost, easy to use Email Server software for virtual, on-premise, or hosted deployments for companies that want control of their email. Integrated web-mail with doc sharing, Drop Box integration, client and server side encryption.
Fast, safe, heavy mail solution. Web-client access, LAN or WAN self-hosted server, maintenance-free design. No Windows server and database server required, unlimited mail accounts, storage, attachment size, custom logo. Free for commercial use.
Sanwhole Exchange is a server running iMail, iPress and iStore services. iMail is an enterprise-class email server does Microsoft Exchange and Gmail work. iPress sets blog, forums and tickets in one. iStore is an online store.
Online Collaboration with Chat and Online Office. Is developed as open source software in Germany, offered as free community version and as commercial version. Operation as self-hosted or as SaaS from the computer centre.