Encryption is the process of encoding messages or information in such a way that only authorized parties can read it. Using complex algorithms, the message is transformed into an illegible string of characters without the uniquely generated key. This unique key allows for the message to be transformed back into its original format.
ProtonMail uses encryption to encrypt and decrypt the messages you send and receive. All messages you send to other ProtonMail users are encrypted in this way. Also when you send messages to other services, if you choose to encrypt the message with a password, the same method is used.
When a ProtonMail user sends a message to another ProtonMail user, that message is encrypted, and can only be decrypted with the recipients key. When the recipient gets the message, it would look like this:
Without the appropriate mailbox password the correct key can’t be used, and you will see this:
The same happens if you reset your mailbox password. New keys are generated, and you will see this decryption error on all old messages.