Purge delete on exit --> only does "local (in app)" folders, i.e. if your Deleted are not set to sync with the server (typical for POP3, not IMAP).
Just to protect certain type of users from themselves and me - from them...
Now on the other hand, if you use Menu -> Purge deleted folder on the app's main screen, then this will include all Deleted folders, set to sync with the server or not.