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Betterbird vs Aerion: catalog facts
Betterbird	Aerion
Free Free and open source, funded by donations.	Free Free and open source under the Apache 2.0 licence.
Windows, macOS, Linux	Windows, macOS, Linux
Fixes and features Thunderbird has not shipped, listed openly in a comparison table Installs next to Thunderbird and runs on the same profile, so switching back is trivial	Built with Wails instead of Electron for lower resource use Direct IMAP and SMTP connection with mail stored locally
A small volunteer project riding on someone else’s release train The Mac build ships in English only and needs a language pack for anything else	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.