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KMail vs Thunderbird: catalog facts
KMail	Thunderbird
Free Free and open source under the GPL.	Free Free forever, funded by donations.
Linux	Windows, macOS, Linux, Android
OpenPGP and S/MIME encryption integrated rather than bolted on Shares contacts and calendar with the rest of the Kontact suite	Unified inbox across every account, or each account kept separate, your choice Mail, calendar, contacts and tasks in one application rather than four
Depends on the Akonadi storage service, which is a common source of problems Linux only in practice	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
Open source	Open source
License: GPL-2.0	License: MPL-2.0
Not stated if an account is needed	No account needed
Not stated if it works offline	Works offline
Official website Full listing and sources Checked September 22, 2026	Official website Full listing and sources Checked September 20, 2026

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