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Thunderbird vs Kontact: catalog facts
Thunderbird	Kontact
Free Free forever, funded by donations.	Free Free and open source, developed by the KDE community.
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	Mail, calendar, contacts and feeds in one window Works offline with your data
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	Best suited to the KDE Plasma desktop Large suite with many settings to learn
Downloadable app	Downloadable app
Open source	Open source
License: MPL-2.0	License not stated
No account needed	No account needed
Works offline	Works offline
Official website Full listing and sources Checked September 20, 2026	Official website Full listing and sources Checked September 24, 2026

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