def do_finish # :nodoc:
      @mails = nil
      @n_mails = nil
      @n_bytes = nil
      @command.quit if @command
    ensure
      @started = false
      @command = nil
      @socket.close if @socket and not @socket.closed?
      @socket = nil
    end
nil's out the:
- mails
- number counter for mails
- number counter for bytes
- quits the current command, if any