To send emails to non-Protonmail users, you must enable the "Encrypt for outside" feature. You can either use the free account or premium account to use ProtonMail. To use the end-to-end encryption, you need to choose a passphrase in your account settings. This password is not saved in the ProtonMail server, but stored on the recipient's computer.
Read also - https://emailsdesk.com/protonmail-com-login/