One of the first things you will notice when logging into ProtonMail, is you are required to have two passwords. While having two passwords increases the security of your account, each of your ProtonMail passwords have their own unique purpose.
The Login Password is used as the first line of defense to ensure no intruders are allowed into your account. This password is stored on ProtonMail servers and allows ProtonMail to confirm your identity.
The Mailbox Password allows you to read your encrypted messages. It is the key that allows you to encrypt and decrypt messages sent to and from your ProtonMail account, ensuring that you are the only one who can read your messages, even if they end up in the wrong hands. This password is not stored on ProtonMail’s servers. It is decrypted within your browser every time you login.