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Rainlendar vs Thunderbird: catalog facts
Rainlendar	Thunderbird
Freemium Lite is free; a Pro licence costs 9.95 EUR.	Free Free forever, funded by donations.
Windows, macOS, Linux	Windows, macOS, Linux, Android
Calendar and tasks always visible on the desktop Skinnable, with a skin tutorial and API reference	Unified inbox across every account, or each account kept separate, your choice Mail, calendar, contacts and tasks in one application rather than four
Some features need the paid Pro edition Closed source	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
Downloadable app	Downloadable app
Closed source	Open source
License not stated	License: MPL-2.0
No account needed	No account needed
Not stated if it works offline	Works offline
Official website Full listing and sources Checked October 2, 2026	Official website Full listing and sources Checked September 20, 2026

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