Encryption is a process of converting messages or information in such a way that only authorized parties can read it. Using complex algorithms, a message is transformed into an illegible string of characters. The only way to transform the message into its original format is to use a unique encryption key.
When you send an email to other Proton Mail users or when you receive a message from another Proton Mail user, the message body and the attachments are fully and automatically encrypted at every step. This is called end-to-end encryption.
All the messages and attachments you receive in your Proton Mail mailbox are also stored with zero-access encryption, including messages from other email providers. Only you have the key to access your messages and files on our secure servers. That means your data isn't accessible to anyone else — not even us.
You can also send end-to-end encrypted messages to people on other email providers using Password-protected Emails.
Otherwise, they will be protected with TLS encryption if the non-Proton Mail server supports it.