Compare software

Microsoft Outlook for iOS and Android vs K-9 Mail: catalog facts
Microsoft Outlook for iOS and Android	K-9 Mail
Freemium The app is a free download; Microsoft 365 plans are sold separately.	Free Free and open source.
Android, iOS	Android
Email, calendar and contacts in one app Available on both iOS and Android phones and tablets	Open source with a long track record on Android Now the foundation for Thunderbird's official mobile app
Closed source Closely tied to Microsoft's services and plans	Android only, no iOS version Interface is more utilitarian than modern polished mail apps
Downloadable app	Downloadable app
Closed source	Open source
License not stated	License: Apache-2.0
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 23, 2026	Official website Full listing and sources Checked September 22, 2026

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