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K-9 Mail vs Gridlink Mail: catalog facts
K-9 Mail	Gridlink Mail
Free Free and open source.	Free Free and open source under GPL-3.0.
Android	Android
Open source with a long track record on Android Now the foundation for Thunderbird's official mobile app	Supports JMAP and classic IMAP and SMTP Includes JMAP contacts and calendar support with CardDAV and CalDAV alternatives
Android only, no iOS version Interface is more utilitarian than modern polished mail apps	Android only A fork of Sterna Mail with a separate release and support path
Downloadable app	Downloadable app
Open source	Open source
License: Apache-2.0	License: GPL-3.0
Not stated if an account is needed	No account 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 8, 2026

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