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

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