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grommunio vs Citadel: catalog facts
grommunio	Citadel
Freemium Open source, with paid plans listed on the pricing page.	Free Free and open source under the GNU GPL, with no paid edition.
Linux, Web, Self-hosted	Web, Self-hosted
Native MAPI, EWS and EAS support for Outlook and mobile clients Mail, calendars, chat, video and files on one platform	Mail, calendar, contacts, forums, wiki and chat in one server Consistent rooms-based structure across features
Running a groupware server requires administration skills Some features may depend on paid plans	The rooms metaphor differs from conventional mail servers Requires running and maintaining your own server
Self-hosted, Hosted service	Self-hosted
Open 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.