@teledyn (Seems my first comment was lost in transaction, so a new try)20 years ago (!), the feature you ask for was already implemented in Opera's built-in email-client. The principle was crazy simple (I guess):One could add folders to the sidebar w...
I was a happy user of the Opera browser with its built in email client back in the days before they dropped support for Linux in 2013.There, one could create a new folder (not physical folder, more the concept of a view I guess), and tick a checkbox ...
