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K-9 Mail vs Ltt.rs: catalog facts
K-9 Mail	Ltt.rs
Free Free and open source.	Free Free and open source under the Apache 2.0 licence.
Android	Android
Open source with a long track record on Android Now the foundation for Thunderbird's official mobile app	Native JMAP support Simple, focused interface
Android only, no iOS version Interface is more utilitarian than modern polished mail apps	Works only with JMAP servers, not IMAP Described as a proof of concept
Downloadable app	Downloadable app
Open source	Open source
License: Apache-2.0	License: Apache-2.0
Not stated if an account is needed	Not stated if an account is needed
Not stated if it works offline	Needs internet
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.