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K-9 Mail vs Spark Mail: catalog facts
K-9 Mail	Spark Mail
Free Free and open source.	Freemium Free for individuals; paid plans add AI features and team tools.
Android	Windows, macOS, Android, iOS
Open source with a long track record on Android Now the foundation for Thunderbird's official mobile app	Automatic inbox categorization cuts down on manual sorting AI-drafted replies can speed up routine email responses
Android only, no iOS version Interface is more utilitarian than modern polished mail apps	AI and team features are gated behind a paid plan Some categorization requires manual correction over time
Downloadable app	Downloadable app
Open source	Closed source
License: Apache-2.0	License not stated
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 September 22, 2026	Official website Full listing and sources Checked September 22, 2026

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