Mail merge from Google Sheets with sent tracking

/**
 * Mail merge from a sheet with sent tracking.
 * Columns: A Email, B Name, C Company, D Status (blank = not sent yet).
 * Template cells: Settings!B1 = subject, Settings!B2 = body. Use {{Name}} and {{Company}}.
 * Run sendMerge from the menu or a trigger. Only rows with a blank Status are sent.
 */
var MM = { dataSheet: 'Sheet1', settingsSheet: 'Settings', statusCol: 4, headerRow: 1, reserve: 0 };

function fill_(template, record) {
  return String(template).replace(/\{\{\s*([^}]+?)\s*\}\}/g, function (m, key) {
    return record[key] === undefined ? m : String(record[key]);
  });
}

function sendMerge() {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var sheet = ss.getSheetByName(MM.dataSheet);
  var settings = ss.getSheetByName(MM.settingsSheet);
  var subject = settings.getRange(1, 2).getValue();
  var body = settings.getRange(2, 2).getValue();
  var values = sheet.getDataRange().getValues();
  var headers = values[0];
  var sent = 0;
  for (var i = MM.headerRow; i < values.length; i++) {
    if (MailApp.getRemainingDailyQuota() <= MM.reserve) break;
    var row = values[i];
    if (row[MM.statusCol - 1]) continue;
    var to = String(row[0]).trim();
    if (!/^[^@\s]+@[^@\s]+\.[^@\s]+$/.test(to)) {
      sheet.getRange(i + 1, MM.statusCol).setValue('Skipped: bad email');
      continue;
    }
    var record = {};
    headers.forEach(function (h, c) { record[h] = row[c]; });
    MailApp.sendEmail(to, fill_(subject, record), fill_(body, record));
    sheet.getRange(i + 1, MM.statusCol).setValue('Sent ' + Utilities.formatDate(new Date(), Session.getScriptTimeZone(), 'yyyy-MM-dd HH:mm'));
    sent++;
  }
  return sent;
}