Enhanced email formatting features
Proton's email formatting features are too limited for Proton to be the world's leading secure email provider.
I suggest that Proton have at look at Zoho's and Gmail's formatting suites. For example, emails look much better and are more readable when the content area is narrowed and centered on the page (needs "deep" indent feature).
Line spacing and undo features are most valuable, too. These are must have basics.

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Wiltshire Bunbury commented
I have a sight disability adjutsment needed and after stuggling for two months with composing emails finding it very hard to read on screen (no ability to enlarge) what I am composing and unable to change the line spacing am likely to have to abandon Protonmail. So much for reasonable adjustment (in UK) under DDA. Really disappointed :-(
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April commented
I second the line spacing!!! And in general better aesthetic formatting features.