Issue: Once you start to have many folders (lets say 100+), it starts getting a bit annoying to find the correct one as you need to scroll down to find it.

Solution: Just above the "Folders" section in the Mail, there should be a text box where you can type, and as you type it filters the folders with those characters.

For example: I want to search for a folder name "Proton". I could just type in this text box "p" "pr" "pro" "prot" "proto" "proton" and find folders that contains these characters. Similar to how Steam does it for the games library.

Bonus 1: Include an toggable option to show the parent folder if the filter finds sub-folders.
Bonus 2: This could also be added to the settings->folders and labels, as well as settings-> filters.

Simple and very useful :)