ProtonMail has an easy built in solution to provide end-to-end encryption for messages sent between ProtonMail Email addresses and Non-ProtonMail Email addresses. These messages do not require the recipient to install additional software or sign up for ProtonMail.
Please note, messages sent encrypted to non-ProtonMail recipients will expire after 28 days by default. You can learn more about expiring messages here.
Here’s how it works:
1. The ProtonMail user composes a message in ProtonMail:
2. Select the Encryption button in the bottom left of the compose box and enter a password and password hint (if desired), and select set.
3. Upon sending the message, a generic message is delivered into the recipients mailbox, providing them with a unique link to open the encrypted message the ProtonMail user has sent. In addition, this message contains the subject of the message, and the unique password hint if the ProtonMail user has designated one.
4. Upon clicking on the link, the recipient is asked to enter the password the ProtonMail user has set in step 1.
5. Upon entering the password, the message will be de-crypted allowing your recipient to view the message and reply securely.
6. Due to the expiration time set to 28 Days by default, after the time elapses the recipient will be shown this message
*Please note- The recipient is not asked to download any software or sign up for ProtonMail at any point in this process. It is up to the ProtonMail user to find the most secure manner to communicate the password they have chosen to protect the encrypted message, to the recipient.
If you are testing this feature with your own email you may be interested in: Outside Encrypted messages not working properly?
If you are sending encrypted email to the same email domain frequently, you may they learn about ProtonMail’s Custom Domain (https://protonmail.com/support/knowledge-base/custom-domains/) offering that allows seamless encryption on any custom domain.