Criptext is a free encrypted email service for Windows, Linux, macOS, Android and iOS. The applications that Criptext uses are open-source.
The encryption service is not cloud-based like Gmail or Outlook. Criptext messages are not stored on the servers that they use permanently, but only stored on the device with end to end encryption. It uses Signal Protocol for encryption.
How does it work;
According to the official website, Criptext sends mail through its servers and deletes everything after the message is delivered. The only exception is when the message cannot be delivered. When for example the recipient's device is offline. The email is stored on the server until it is delivered. If it is not delivered within 30 days, it is also deleted from the server.
Criptext states that each email is encrypted with a unique key. Attachments if any are also encrypted. All data is stored on Amazon Web Services servers, but the encryption keys are stored on users' devices.
It should be noted that the service only encrypts messages sent by one Criptext user to someone else using the same service. So if you want to send messages to other services (Gmail, Outlook, etc.), they won't be encrypted.
To use the Criptext service you need to install a desktop or mobile application and use it to sign up. Once you do, the app will start generating encryption / decryption keys for your account on that device.
You can optionally enter a recovery email address when you sign up. It is only needed to reset your account password and not for any authentication.