addTo(to, cc, bcc) {
    if (
      typeof to === 'undefined' &&
      typeof cc === 'undefined' &&
      typeof bcc === 'undefined'
    ) {
      throw new Error('Provide at least one of to, cc or bcc');
    }
    this.addPersonalization(new Personalization({to, cc, bcc}));
  }
Convenience method for quickly creating personalizations