getRecipientsEmailMap_(recipients) {
    var map = {};
    goog.array.forEach(recipients, function(recipient) {
      var email = utils.text.extractValidEmail(recipient);
      if (email) {
        if (!map.hasOwnProperty(email)) {
          map[email] = [];
        }
        map[email].push(recipient);
      }
    });
    return map;
  }
Extracts user addresses from user IDs and creates an email to user IDs map.
Ignores user IDs without a valid e-mail address.
@param  {!Array.<string>} recipients user IDs of recipients
@return {!Object.<string, !Array.<string>>} email to user IDs map
@private