code: func (h *SMTPAPIHeader) AddFilter(filter, setting string, value interface{}) {
  	if h.Filters == nil {
  		h.Filters = make(map[string]Filter)
  	}
  	if _, ok := h.Filters[filter]; !ok {
  		h.Filters[filter] = Filter{
  			Settings: make(map[string]interface{}),
  		}
  	}
  	h.Filters[filter].Settings[setting] = value
  }
docs: // AddFilter will set the specific setting for a filter