https://rein.kr/posts/2009-06-12-%EC%8B%A4%ED%8C%A8%ED%95%A0-%EC%9D%B8%ED%84%B0%EB%84%B7-%EA%B4%91%EA%B3%A0-%ED%85%8C%EB%A0%88%EB%B9%84-a-k-a-spam/