> This option is really simple: It allows us to override automatic sorting for certain folders. The specified folders are shown at the top of the folder list in the provided order. The rest are sorted alphabetically. This option should be in every email client. Sadly, I’ve never seen it in a graphical client (Thunderbird/Outlook) before.

Isn’t this just favorites? I’ve used them for over a decade in Outlook to keep some folders above others. It’s not configurable by a file, sure, but the feature is there.