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Apache James vs Citadel: catalog facts
Apache James	Citadel
Free Free and open source from the Apache Software Foundation.	Free Free and open source under the GNU GPL, with no paid edition.
Self-hosted	Web, Self-hosted
Supports modern JMAP alongside SMTP, IMAP and POP3 Mailet container for custom mail processing	Mail, calendar, contacts, forums, wiki and chat in one server Consistent rooms-based structure across features
Requires a Java runtime and some server administration knowledge Distributed setups depend on extra services such as Cassandra	The rooms metaphor differs from conventional mail servers Requires running and maintaining your own server
Self-hosted	Self-hosted
Open source	Open source
License: Apache-2.0	License: GPL
No account 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 2, 2026

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