Long time Fastmail customer here. They need to stop their recent trend of breaking their UI, especially in backwards-incompatible ways.

They recently recently removed app-based font size from the backend (but just for mobile), and released an updated Android app which only uses the system-level font scaling, which broke it for people who hadn't upgraded yet. Plus the app-based functionality was useful. Not sure why couldn't just add a new "Use system font-scaling"

Then there was the debacle of removing horizontal separators in the list view. After backlash, they added an option.