How Proton Mail messages are encrypted

All messages and attachments in your Proton Mail inbox are secured with zero-access encryption. That means nobody else can access your emails, not even Proton. Messages are encrypted in transit too, but the type of encryption depends on the recipient’s email provider. 

Learn more about encryption

In your inbox

All messages in your Proton Mail inbox are stored with zero-access encryption.

• Body content and attachments are stored with end-to-end encryption.
• Subject lines, recipient email addresses, and sender email addresses are encrypted, but not end-to-end encrypted. Learn more

This includes messages sent from non-Proton Mail addresses.

Sending and receiving messages

Between Proton Mail addresses

Messages in transit are always end-to-end encrypted. 

From Proton Mail to non-Proton Mail addresses

Messages in transit are encrypted with TLS. Most third-party email providers support this.

Once the email reaches your recipient, it isn’t end-to-end encrypted by default. So the recipient’s email provider may be able to access it.

To send end-to-end encrypted messages to a non-Proton Mail address, use our password-protected emails feature.

From non-Proton Mail to Proton Mail addresses

Messages are encrypted with TLS in transit, then stored on our servers with zero-access encryption.

Unless the sender’s email provider uses end-to-end encryption, they may be able to retain a copy of messages sent from their side. 

To receive end-to-end encrypted messages from a contact who doesn’t use Proton, you can:  

• Send them a password-protected email and ask them to reply to it.
• Ask them to use PGP. How to export your Proton Mail PGP key 

Alternatively, consider asking them to create a free Proton Mail email address, so your communications stay private by default.

A note about email subject lines

Subject lines in Proton Mail messages are not end-to-end encrypted to remain compliant with standards and ensure interoperability. Proton Mail adheres to the OpenPGP standard. In PGP, the subject line is part of the header packet, which is not end-to-end encrypted.

Even though Proton Mail subject lines are not end-to-end encrypted, it is exceptionally difficult for a third party to get access to them. Access would require a court order that is approved by a Swiss judge.

You can also use generic subject lines that disclose minimal information about the message contents.