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