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Proton Mail Discloses User Data Leading to Arrest in Spain
(restoreprivacy.com)
This is a most excellent place for technology news and articles.
Proton's mails are encrypted... between proton accounts. Send an email to a hotmail account and bye-bye encryption. Proton does rely on PGP so you can use that if the recipient supports it.
They mean encrypted at rest. As in, Proton cannot hand over a copy of all your emails to a law enforcement agency, they don’t have access.
This means law enforcement would have to capture an unencrypted email in transit, or obtains your emails from either recipient individually.
Mail stored in proton is encrypted
https://proton.me/support/password-protected-emails