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Claws Mail vs Aerion: catalog facts
Claws Mail	Aerion
Free Free and open source under the GPL 3.0 or later.	Free Free and open source under the Apache 2.0 licence.
Windows, macOS, Linux	Windows, macOS, Linux
Opens and searches enormous mail folders without stalling Plugins for PGP, spam filtering, RSS and calendars ship with it	Built with Wails instead of Electron for lower resource use Direct IMAP and SMTP connection with mail stored locally
Composes plain text by default; HTML mail sending is deliberately limited The interface is functional and looks like it was designed in 2004	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: GPL-3.0-or-later	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 21, 2026	Official website Full listing and sources Checked October 1, 2026

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