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Thunderbird vs Spark Mail: catalog facts
Thunderbird	Spark Mail
Free Free forever, funded by donations.	Freemium Free for individuals; paid plans add AI features and team tools.
Windows, macOS, Linux, Android	Windows, macOS, Android, iOS
Unified inbox across every account, or each account kept separate, your choice Mail, calendar, contacts and tasks in one application rather than four	Automatic inbox categorization cuts down on manual sorting AI-drafted replies can speed up routine email responses
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	AI and team features are gated behind a paid plan Some categorization requires manual correction over time
Downloadable app	Downloadable app
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 22, 2026

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