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Thunderbird vs FMail3: catalog facts
Thunderbird	FMail3
Free Free forever, funded by donations.	Free Downloadable from the developer's site; the homepage does not list a price.
Windows, macOS, Linux, Android	macOS
Unified inbox across every account, or each account kept separate, your choice Mail, calendar, contacts and tasks in one application rather than four	Native macOS windows and tabs with session restore Not built on Electron
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	Works only with Fastmail accounts Unofficial and not made by Fastmail
Downloadable app	Downloadable app
Open source	Closed source
License: MPL-2.0	License not stated
No account needed	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.