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Claws Mail vs KMail: catalog facts
Claws Mail	KMail
Free Free and open source under the GPL 3.0 or later.	Free Free and open source under the GPL.
Windows, macOS, Linux	Linux
Opens and searches enormous mail folders without stalling Plugins for PGP, spam filtering, RSS and calendars ship with it	OpenPGP and S/MIME encryption integrated rather than bolted on Shares contacts and calendar with the rest of the Kontact suite
Composes plain text by default; HTML mail sending is deliberately limited The interface is functional and looks like it was designed in 2004	Depends on the Akonadi storage service, which is a common source of problems Linux only in practice
Downloadable app	Downloadable app
Open source	Open source
License: GPL-3.0-or-later	License: GPL-2.0
No account needed	Not stated if an account is needed
Works offline	Not stated if it works offline
Official website Full listing and sources Checked September 21, 2026	Official website Full listing and sources Checked September 22, 2026

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