I've been using protonmail as my mail email service for probably close to 6 years. Earlier iterations of the UI were obviously basic but it's perfectly functional now and works very well and certainly not for "occasional use". The web client UI is great and has come on leaps and bounds particularly of late.
The mobile app has some way to go but more than adequate for daily use because I'm using it daily.
The mobile app has some way to go but more than adequate for daily use because I'm using it daily.