Setting Protonmail on default email mobile phone app
Protonmail does not work on Android in third party Mail applications (because the ProtonMail Bridge application is not available for Android). You have to use the web interface or the ProtonMail app.

Cite from the ProtonMail help pages:


  At this time Protonmail does not integrate with third party email clients (IMAP/SMTP) on Android due to the technology ProtonMail utilizes within web browsers to encrypt and decrypt your messages. [source]