A mail::folder object represents a folder that contains messages and/or other folders. The folders are arranged in a top-to-bottom hierarchy. mail::account::readTopLevelFolders(3x) enumerates mail::folder objects at the top of the folder hierarchy. mail::folder::readSubFolders(3x) enumerates each mail::folder object's subfolders.
Like mail::account's
      methods, mail::folder's
      methods receive a mail::callback object, and invoke its
      success or fail method when the request is processed.
      mail::folder's methods return
      immediately, even if the request cannot be processed right
      away, and the mail::account::process(3x)
      method will invoke success or
      fail method at some later point
      in time.