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K-9 Mail vs Microsoft Outlook for iOS and Android: catalog facts
K-9 Mail	Microsoft Outlook for iOS and Android
Free Free and open source.	Freemium The app is a free download; Microsoft 365 plans are sold separately.
Android	Android, iOS
Open source with a long track record on Android Now the foundation for Thunderbird's official mobile app	Email, calendar and contacts in one app Available on both iOS and Android phones and tablets
Android only, no iOS version Interface is more utilitarian than modern polished mail apps	Closed source Closely tied to Microsoft's services and plans
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 23, 2026

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