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FMail3 vs Thunderbird: catalog facts
FMail3	Thunderbird
Free Downloadable from the developer's site; the homepage does not list a price.	Free Free forever, funded by donations.
macOS	Windows, macOS, Linux, Android
Native macOS windows and tabs with session restore Not built on Electron	Unified inbox across every account, or each account kept separate, your choice Mail, calendar, contacts and tasks in one application rather than four
Works only with Fastmail accounts Unofficial and not made by Fastmail	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
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.