Citadel

citadel.org · checked 30 Sept 2026

#11 · Mail Server Software

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How it plugs in

—Public API

NoOpen source

YesFree tier

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Citadel is an email, collaboration, groupware, and content-management system.

Pricing

Citadel software100% open source · GNU General Public License · self-hosted or cloud deploymentFree

As citadel.org lists them, checked 30 Sept 2026.

Near it

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Runs on

Web
Windows
Mac
Linux
iOS
Android

Facts

Website: citadel.org
Price: Free tier
Free trial: Not stated
Architecture: Citadel uses a rooms-based architecture for organizing collaboration features.
Authentication: Citadel can authenticate users through its own database, host-system accounts, or LDAP including Microsoft Active Directory schemas.
Calendar integration: Calendar and address-book functions are available to PIM clients supporting Webcal or GroupDAV.
Core features: The system includes email, calendars, contacts, forums, mailing lists, instant messaging, wikis, blogs, and RSS aggregation.
Deployment: Citadel can run in Docker, through a Linux build script, from source, or via a hosting provider.
Docker platforms: The Docker image offers AMD64, i386, ARMv7, and ARM64 builds.
Email clients: Citadel provides built-in SMTP, POP, and IMAP services and works with standards-compliant clients such as Thunderbird and Outlook.
Encryption: Citadel provides TLS encryption for ESMTP, IMAP, POP3, and the Citadel client protocol.
History: Citadel began in December 1981 as a dial-up BBS package written by Jeff Prothero for 8-bit CP/M systems.
Message journaling: Citadel journaling records processed messages but is not itself an archival service, so the project recommends pairing it with a third-party archival product or service.
Support: Support is provided through an active user and developer community, an online knowledge base, a support forum, and a mailing list.
System requirements: Manual installation requires a modern Unix operating system, root access, Berkeley DB 5 or newer, and libical 3 or newer.

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