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