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K-9 Mail vs FairEmail: catalog facts
K-9 Mail	FairEmail
Free Free and open source.	Free Free to download from Google Play, F-Droid or GitHub releases.
Android	Android
Open source with a long track record on Android Now the foundation for Thunderbird's official mobile app	Works with nearly all IMAP providers and unlimited accounts OpenPGP and S/MIME encryption built in
Android only, no iOS version Interface is more utilitarian than modern polished mail apps	No Microsoft Exchange Web Services or ActiveSync support Android only
Downloadable app	Downloadable app
Open source	Open source
License: Apache-2.0	License: GPL-3.0
Not stated if an account is needed	Not stated if an account is 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 23, 2026

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