docs: public void initializeSendGrid(String apiKey) {
      this.apiKey = apiKey;
      this.host = "api.sendgrid.com";
      this.version = "v3";
      this.requestHeaders = new HashMap<String, String>();
      this.requestHeaders.put("Authorization", "Bearer " + apiKey);
      this.requestHeaders.put("User-agent", USER_AGENT);
      this.requestHeaders.put("Accept", "application/json");
      this.rateLimitRetry = 5;
      this.rateLimitSleep = 1100;
  
      this.pool = Executors.newFixedThreadPool(THREAD_POOL_SIZE);
    }