At this time Protonmail does not support IMAP/SMTP or POP3 due to the technology ProtonMail utilizes within web browsers to encrypt and decrypt your messages. We apologize for the inconvenience and are working on creative solutions to allow IMAP/SMTP use.
There is an option to be a Beta tester still, so if you want to gain early access to them as Beta Tester (get advanced updates etc), please follow this link: https://protonmail.com/mobile-invite.
You can access your ProtonMail account through modern browsers on most PCs and Mobile devices.
Up-vote this article if you want this feature now (comment if you have an opinion on the topic)!