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Mailspring vs Aerion: catalog facts
Mailspring	Aerion
Freemium The client is free and open source. Mailspring Pro is a monthly subscription that adds…	Free Free and open source under the Apache 2.0 licence.
Windows, macOS, Linux	Windows, macOS, Linux
Unified inbox, snooze, send later, rules and templates in the free tier C++ sync engine keeps memory and battery use down for an Electron app	Built with Wails instead of Electron for lower resource use Direct IMAP and SMTP connection with mail stored locally
Read receipts and link tracking are paid, and are tracking features Electron, so still heavier than a native client	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	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.