Gmail: Automatically send email on trigger

sendOnTrigger.js

/**
 * Sends an email when a trigger is received.
 */
function RespondEmail(e) {

  // All messages marked with this label will trigger a new email to be sent
  var triggerLabel = "paymenttrigger";

  // The message content is pulled from a saved draft email. draftSubject
  // specifies the subject to search for.
  var draftSubject = "Your ebook is here!";

  // The name to reply as
  var replyFrom = '"My Name" <myname@gmail.com>';


  /**
   * Loads the message from the saved draft with the specified subject.
   *
   * @param {string} - The subject of the draft to retrieve the message from.
   * @return {GmailMessage} - A Gmail message loaded from the draft.
   */
  var getDraftMessage = (function(draftSubject) {
    var message = "";

    // Cache the message so we only have to search for the draft once
    return function(draftSubject) {
      if (!message) {

        // Loop through all saved drafts
        var drafts = GmailApp.getDrafts();
        for (var i = 0; i < drafts.length; ++i) {
          var draft = GmailApp.getDraft(drafts[i].getId());

          // End search if we've found the subject we're looking for
          if (draft.getMessage().getSubject() == draftSubject) {
            message = draft.getMessage();
            break;
          }
        }

        if (!message) {
          throw 'Cannot find draft with subject "' + draftSubject + '"';
        }
      }

      return message;
    };
  }());


  /**
   * Extracts the email address from the message.
   *
   * @param {string} - The message contents to extract the email address
   * from.
   * @return {string} - The extracted email address.
   */
  var extractEmailFromMessage = function(message) {
    var matches = message.match(/You received a payment[ $.\w]+\(([^)]+)\)/im);

    if (!matches) {
      throw 'Cannot find email address inside "' + message + '"';
    }

    return matches[1];
  };

  /**
   * Sends a message based on the given GmailMessage.
   *
   * @param {string} - The email address to send the message to.
   * @param {GmailMessage} - The GmailMessage to send.
   */
  var sendMessage = function(to, message) {
    GmailApp.sendEmail(to, message.getSubject(), message.getPlainBody(), {
      attachments: message.getAttachments(),
      from: replyFrom,
      htmlBody: message.getBody()
    });
  };


  // Search for all messages marked with our label
  var trigger = GmailApp.getUserLabelByName(triggerLabel);
  var messages = GmailApp.search("label:" + triggerLabel);

  // Send out a new email for all the messages we found. Make sure to remove
  // the trigger label so the messages aren't picked up next time.
  for (var i = 0; i < messages.length; ++i) {
    var sendTo = extractEmailFromMessage(messages[i].getMessages()[0].getPlainBody());
    sendMessage(sendTo, getDraftMessage(draftSubject));
    messages[i].removeLabel(trigger);
  }
}