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Betterbird vs Claws Mail: catalog facts
Betterbird	Claws Mail
Free Free and open source, funded by donations.	Free Free and open source under the GPL 3.0 or later.
Windows, macOS, Linux	Windows, macOS, Linux
Fixes and features Thunderbird has not shipped, listed openly in a comparison table Installs next to Thunderbird and runs on the same profile, so switching back is trivial	Opens and searches enormous mail folders without stalling Plugins for PGP, spam filtering, RSS and calendars ship with it
A small volunteer project riding on someone else’s release train The Mac build ships in English only and needs a language pack for anything else	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: MPL-2.0	License: GPL-3.0-or-later
No account needed	No account needed
Works offline	Works offline
Official website Full listing and sources Checked September 20, 2026	Official website Full listing and sources Checked September 21, 2026

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