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KMail vs Mailspring: catalog facts
KMail	Mailspring
Free Free and open source under the GPL.	Freemium The client is free and open source. Mailspring Pro is a monthly subscription that adds…
Linux	Windows, macOS, Linux
OpenPGP and S/MIME encryption integrated rather than bolted on Shares contacts and calendar with the rest of the Kontact suite	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
Depends on the Akonadi storage service, which is a common source of problems Linux only in practice	Read receipts and link tracking are paid, and are tracking features Electron, so still heavier than a native client
Downloadable app	Downloadable app
Open source	Open source
License: GPL-2.0	License: GPL-3.0
Not stated if an account is needed	No account needed
Not stated if it works offline	Works offline
Official website Full listing and sources Checked September 22, 2026	Official website Full listing and sources Checked September 21, 2026

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