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Thunderbird vs Microsoft Outlook: catalog facts
Thunderbird	Microsoft Outlook
Free Free forever, funded by donations.	Freemium Free app and account available; Microsoft 365 plans add paid features and Office apps.
Windows, macOS, Linux, Android	Windows, macOS, Android, iOS, Web
Unified inbox across every account, or each account kept separate, your choice Mail, calendar, contacts and tasks in one application rather than four	Email, calendar and contacts for multiple accounts in one app Available on desktop, mobile and web
The interface carries a lot of history and can feel dense next to a webmail tab Major ESR releases have historically been disruptive for add-ons and settings	Closed source and tied to Microsoft services Some features, including Copilot, require a paid plan
Downloadable app	Downloadable app, Web application
Open source	Closed source
License: MPL-2.0	License not stated
No account needed	Not stated if an account is needed
Works offline	Not stated if it works offline
Official website Full listing and sources Checked September 20, 2026	Official website Full listing and sources Checked September 23, 2026

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