ProtonMail's verification requirements are determined by a complex system of IP reputation and other factors that are analysed in realtime when the sign up page

ProtonMail's verification requirements are determined by a complex system of IP reputation and other factors that are analysed in realtime when the sign up page is rendered. As an e-mail service, one of the most challenging things for ProtonMail is preventing abusive accounts from damaging the service's SMTP sending IP addresses' reputation to the point where deliver-ability becomes impossible with other e-mail service providers. This is especially difficult for e-mail providers that work to deliver privacy, and potentially pseudoanonymity, to users.