Google Apps Script Mass Mailer: always the latest version at https://script.google.com/macros/s/AKfycbyn1qAemrlvvI8QVl2Xgdjs-2E8l5g3vQQaXae0RTAab4xewP0/exec

mass_emailer.js

/*
 * Mass Emailer Google Spreadsheet script.
 *
 * Changelog:
 * 
 *  27. Apr. 2013
 *   - trial checkbox allows easier debugging
 *  16. Feb. 2012
 *    - released the first version to public  
 *  15. Feb. 2012
 *   - add possibility to change the name
 *   - add full-blown UI
 *  10. Feb. 2012
 *   - add Support to load Mail from Draft
 */
function start_mailer() {
  var myapp = UiApp.createApplication().setTitle('Mass Emailer').setHeight(150).setWidth(300);
  var top_panel = myapp.createFlowPanel(); 
  
  top_panel.add(myapp.createLabel("Please select the draft you'd like to be sent"));

  var lb = myapp.createListBox(false).setId('msg').setWidth(300).setName('message').setVisibleItemCount(1);
 
  var threads = GmailApp.search('is:draft', 0, 10);
  if (threads.length === 0) {
    Browser.msgBox("Please save your template as a draft in your gmail account!");
    return;
  }
  
  for (var i = 0; i < threads.length; i++) {
    lb.addItem("" + threads[i].getFirstMessageSubject(), threads[i].getMessages()[0].getId());
  }
  
  top_panel.add(lb);

  
  top_panel.add(myapp.createLabel("Name to send from (optional)"));
  var name_box = myapp.createTextBox().setName("name").setWidth(300);
  top_panel.add(name_box);
  
  var check_box = myapp.createCheckBox().setName("trial").setText("Trail Run").setWidth(300);
  top_panel.add(check_box);
  
  var ok_btn = myapp.createButton('Send mails now'); 
  top_panel.add(ok_btn);
  myapp.add(top_panel);
  
  var handler = myapp.createServerClickHandler('callback').addCallbackElement(lb).addCallbackElement(name_box).addCallbackElement(check_box);
  ok_btn.addClickHandler(handler);
  
  SpreadsheetApp.getActiveSpreadsheet().show(myapp);
}

function callback(e)  {
  var mail = GmailApp.getMessageById(e.parameter.message);
  _send_mails(mail, e.parameter.name, e.parameter.trial);
  var app = UiApp.getActiveApplication();
  app.close();
  return app;
  
  }

function _send_mails(mail, name, trial) {
  var ws = SpreadsheetApp.getActiveSpreadsheet().getActiveSelection(),
      data = ws.getValues(),
      attrs = data.shift(),
      count = 0,
      mail, bodyCopy, attachments, subjectCopy, idx, line_idx, mail_idx, line;
  mail_idx = attrs.indexOf('Email');
  if (mail_idx === -1) {
    Browser.msgBox("Canceled: At least one row must be called 'email'");
    return;
  }

  attachments = mail.getAttachments();


  for (line_idx in data) {
    line = data[line_idx];
    bodyCopy = mail.getBody();
    subjectCopy = mail.getSubject();
    for (idx in attrs) {
      bodyCopy = bodyCopy.replace("{{" + attrs[idx] + "}}", line[idx]);
      subjectCopy = subjectCopy.replace("{{" + attrs[idx] + "}}", line[idx]);
    }
    count += 1;
    if(trial == "true") {
      Browser.msgBox("to:" + line[mail_idx] + "\nSubject:" + subjectCopy + "\nBody:" + bodyCopy);
      return;
    } else {
      GmailApp.sendEmail(line[mail_idx], subjectCopy, bodyCopy, {
        htmlBody: bodyCopy,
        name: name,
        attachments: attachments
      });
    }
  }
  
  Browser.msgBox(count + " Mails send");
}



function onOpen() {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var menuEntries = [ {name: "Send Mass Email", functionName: "start_mailer"}];
  ss.addMenu("Mass-Mailer", menuEntries);
}

Sorry, it is broken, since UIapp is unsupported anymore. Must be replaced with HTML service...