code: func NewPersonalization() *Personalization {
  	return &Personalization{
  		To:                  make([]*Email, 0),
  		CC:                  make([]*Email, 0),
  		BCC:                 make([]*Email, 0),
  		Headers:             make(map[string]string),
  		Substitutions:       make(map[string]string),
  		CustomArgs:          make(map[string]string),
  		DynamicTemplateData: make(map[string]interface{}),
  		Categories:          make([]string, 0),
  	}
  }
docs: // NewPersonalization ...