Running 10.12.1 on an 2015 iMac 27". Mail no longer opens enclosing folders when an email, from the inbox, is dragged over the enclosing folder. Oddly enough my 2011 macbook pro, running the same OS, still has this functionality. Anyone else seeing this problem on their machine? My fix for now is to leave my main two enclosing folders open so that their subfolders are always visible.