https://blog.toright.com/posts/5256/%e6%95%b4%e5%90%88-slack-%e7%99%bc%e9%80%81%e8%a8%8a%e6%81%af%e5%8f%96%e4%bb%a3%e5%82%b3%e7%b5%b1-email-%e9%80%9a%e7%9f%a5