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Citadel vs Courier Mail Server: catalog facts
Citadel	Courier Mail Server
Free Free and open source under the GNU GPL, with no paid edition.	Free Free and open-source software.
Web, Self-hosted	Linux, Self-hosted
Mail, calendar, contacts, forums, wiki and chat in one server Consistent rooms-based structure across features	SMTP, IMAP, POP3, webmail and mailing lists in one package Components can be switched on or off individually
The rooms metaphor differs from conventional mail servers Requires running and maintaining your own server	Built from source on POSIX systems, with no Windows support Calendaring in the webmail is basic
Self-hosted	Self-hosted
Open source	Open source
License: GPL	License not stated
Not stated if an account is needed	No account 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.