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Mailvelope vs Gpg4win: catalog facts
Mailvelope	Gpg4win
Free Free and open source for personal use. Business products are paid.	Free Free software, funded by donations and public contracts.
Browser extension	Windows
Works inside existing webmail rather than requiring a desktop mail client Keys stay in the browser under your control	Kleopatra gives GnuPG a real key manager instead of a command line Integrates with Outlook through GpgOL and with Explorer through GpgEX
Only supports a defined list of webmail sites Browser-based key storage is weaker than a dedicated client or smartcard	Windows only OpenPGP key management is genuinely hard, and the concepts do not hide well behind a GUI
Browser extension	Downloadable app
Open source	Open source
License: AGPL-3.0	License: GPL-2.0
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 20, 2026

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