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ThunderbirdK-9 Mail
Free

Free forever, funded by donations.

 Free

Free and open source.

Windows, macOS, Linux, Android Android
  • Unified inbox across every account, or each account kept separate, your choice
  • Mail, calendar, contacts and tasks in one application rather than four
  • Open source with a long track record on Android
  • Now the foundation for Thunderbird's official mobile app
  • 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
  • Android only, no iOS version
  • Interface is more utilitarian than modern polished mail apps
Downloadable app Downloadable app
Open source Open source
License: MPL-2.0 License: Apache-2.0
No account needed Not stated if an account is needed
Works offline Not stated if it works offline

Checked September 20, 2026

Checked September 22, 2026

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