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Thunderbird vs KMail: catalog facts
Thunderbird	KMail
Free Free forever, funded by donations.	Free Free and open source under the GPL.
Windows, macOS, Linux, Android	Linux
Unified inbox across every account, or each account kept separate, your choice Mail, calendar, contacts and tasks in one application rather than four	OpenPGP and S/MIME encryption integrated rather than bolted on Shares contacts and calendar with the rest of the Kontact suite
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	Depends on the Akonadi storage service, which is a common source of problems Linux only in practice
Downloadable app	Downloadable app
Open source	Open source
License: MPL-2.0	License: GPL-2.0
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.