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K-9 Mail vs BlueMail: catalog facts
K-9 Mail	BlueMail
Free Free and open source.	Freemium The client is free to download. Some features are reserved for paid plans.
Android	Windows, macOS, Linux, iOS
Open source with a long track record on Android Now the foundation for Thunderbird's official mobile app	One inbox covers many accounts across Windows, macOS, Linux, Android and iOS with a consistent interface and shared…
Android only, no iOS version Interface is more utilitarian than modern polished mail apps	It is closed source, some features need a paid plan, and its handling of account credentials has drawn…
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.