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Zimbra vs Citadel: catalog facts
Zimbra	Citadel
Paid Sold in several editions under Zimbra's license terms; see the edition comparison.	Free Free and open source under the GNU GPL, with no paid edition.
Web, Self-hosted	Web, Self-hosted
Mail, calendar and contacts in one web client Can be run on your own servers	Mail, calendar, contacts, forums, wiki and chat in one server Consistent rooms-based structure across features
Aimed at organisations rather than individuals Running a mail server takes administration effort	The rooms metaphor differs from conventional mail servers Requires running and maintaining your own server
Self-hosted, Hosted service	Self-hosted
Closed source	Open source
License not stated	License: GPL
Not stated if an account is needed	Not stated if an account is needed
Not stated if it works offline	Not stated if it works offline
Official website Full listing and sources Checked October 1, 2026	Official website Full listing and sources Checked October 2, 2026

Catalog facts only. Anything “not stated” is unconfirmed. Check full listings for details.