If you have folders in your folder list that are in italics and cannot be deleted, this due to one of two causes:
1) The folder is not subscribed to, but it does contain sub-folders that are subscribed to. To fix this, subscribe to the folder by going to the File menu > Subscribe > select the account > then select the folder and subscribe to it.
Restart Postbox after this is done to reset the folder list.
2) The folder is locked on the server. In this case, you must contact your mail provider and have the folder unlocked.