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

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