# File lib/sup/mbox/loader.rb, line 58
  def load_header offset
    header = nil
    @mutex.synchronize do
      @f.seek offset
      l = @f.gets
      unless MBox::is_break_line? l
        raise OutOfSyncSourceError, "mismatch in mbox file offset #{offset.inspect}: #{l.inspect}." 
      end
      header = parse_raw_email_header @f
    end
    header
  end