    # File bin/experimental-deliver.rb, line 86
    def filter_pid(queue_dir)
      pid = nil

      begin
        File::open("Maildir/.filterpid") { |file|
          if Time.now - file.stat.mtime < (60 * 60 * 4)
            pid_str = file.gets.chomp.strip
            if pid_str =~ /^\d+/
              pid = Integer(pid_str)
            end
          end
        }
      rescue Errno::ENOENT
      end
      return pid
    end