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K-9 Mail vs Sterna Mail: catalog facts
K-9 Mail	Sterna Mail
Free Free and open source.	Free Free and open source under the GPL-3.0-or-later licence, with no ads.
Android	Android
Open source with a long track record on Android Now the foundation for Thunderbird's official mobile app	Native JMAP support as well as IMAP/SMTP No tracking, ads or Google services
Android only, no iOS version Interface is more utilitarian than modern polished mail apps	Android only Young project still changing its interface often
Downloadable app	Downloadable app
Open source	Open source
License: Apache-2.0	License: GPL-3.0-or-later
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 October 2, 2026

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