Update: As of December 2016, Proton Mail has switched to a single password system as the default. The two-password mode described in this article is still available and supported, but is no longer the default.

Proton Mail uses two passwords to secure your account: login and mailbox.

Login password

Your Login password is the first password you enter and it authenticates you and logs you into our secure system.(new window)
Your login password is the first password you enter and it authenticates you and logs you into our secure system.

MailBox password

Your Mailbox password is the second password you enter and this unlocks and decrypts your encrypted messages locally.(new window)
Your mailbox password is the second password you enter and this unlocks and decrypts your encrypted messages locally.