NotificateToSlack.js

var slackUrl = "WEBHOOK URL"
var selfAddress = "YOUR EMAIL ADDRESS FOR ERROR NOTIFICATION";
var defaultPayload = {
  "channel": "notification",
  "username": "ヤマト不在通知",
  "icon_emoji": ":smiley_cat:"
};

function clone(obj) { 
  if (null == obj || "object" != typeof obj) return obj; 
  var copy = obj.constructor(); 
  for (var attr in obj) { 
    if (obj.hasOwnProperty(attr)) copy[attr] = obj[attr]; 
  } 
  return copy; 
} 

function postSlack(message) {
  var payload = clone(defaultPayload);
  payload['text'] = message.getPlainBody();
      
  var payloadStr = JSON.stringify(payload);
  // :は\:にエスケープする必要がある.
  var escapedStr = payloadStr.replace(/":"/g, "\"\:\"");
  Logger.log(escapedStr);
  
  var options = {
    'method' : 'post',
    'contentType' : 'Content-type: application/json; charset=utf-8',
    'payload' : escapedStr
  }

  var response = UrlFetchApp.fetch(slackUrl, options);

  var ret = true;
  if (response.getResponseCode() != 200) {
    ret = false;
  }
  return {status: ret, code: response.getResponseCode(), response: response};
}

function postError(message) {
  GmailApp.sendEmail(selfAddress, "Slackへのポストに失敗しました", "スクリプトを確認してください。失敗メールの件名：" + message.getSubject());
}

function messageFilter(message) {
  var sub = message.getSubject();
  if (sub.includes("ご不在連絡ｅメール")) {
    return message;
  }
  return null;
}

function main() {
  var searchQuery = 'from:mail@kuronekoyamato.co.jp'
  var threads = GmailApp.search(searchQuery);

  for (var i = 0; i < threads.length; i++) {
    var thread = threads[i];
    if (!thread.isUnread()) {
      continue;
    }

    var messages = thread.getMessages();
    for (var j = 0; j < messages.length; j++) {
      var message = messageFilter(messages[j]);
      if (!message || !message.isUnread()) {
        continue;
      }
      res = postSlack(message);
      
      if (!res.status) {
        postError(message);
        continue;
      }
      
      message.markRead();
    }
    thread.markRead();  
  }
}