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Thunderbird vs Rainlendar: catalog facts
Thunderbird	Rainlendar
Free Free forever, funded by donations.	Freemium Lite is free; a Pro licence costs 9.95 EUR.
Windows, macOS, Linux, Android	Windows, macOS, Linux
Unified inbox across every account, or each account kept separate, your choice Mail, calendar, contacts and tasks in one application rather than four	Calendar and tasks always visible on the desktop Skinnable, with a skin tutorial and API reference
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	Some features need the paid Pro edition Closed source
Downloadable app	Downloadable app
Open source	Closed source
License: MPL-2.0	License not stated
No account needed	No account 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 October 2, 2026

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