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Thunderbird vs Aerion: catalog facts
Thunderbird	Aerion
Free Free forever, funded by donations.	Free Free and open source under the Apache 2.0 licence.
Windows, macOS, Linux, Android	Windows, macOS, Linux
Unified inbox across every account, or each account kept separate, your choice Mail, calendar, contacts and tasks in one application rather than four	Built with Wails instead of Electron for lower resource use Direct IMAP and SMTP connection with mail stored locally
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	Young project with a smaller feature set than established clients Tracking protection is described as basic
Downloadable app	Downloadable app
Open source	Open source
License: MPL-2.0	License: Apache-2.0
No account needed	No 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 1, 2026

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