Where are my email folders / labels?
Missive fully supports folders and sub-folders (they are called Labels in the Missive interface). You just need to open the More menu in the left sidebar to list all your available labels. When you click on a label from there, it will be opened and pinned to the sidebar. You can also choose to open a label, but hide it unless it contains unread content, as shown here in the right-click menu: