In High Sierra’s Mail you can change the order of mail folders within a certain account in the Mailboxes list. The default is alphabetical, but you can then click and drag to rearrange in any way that suits you.
If you then, however, use menus to move email for example, the folders always get listed alphabetically and never in the order you chose. You can see this behavior in the Copy To… or Move To… menus as well as if you use the Move icon in the toolbar.
Is this intentional or a bug? I find it rather strange that you first indicate the user can customize, but then later do not follow the user’s selection.