Запись полученных ответов от SMTP сервера в файл (cURL)

#include <stdio.h>
#include <string.h>
#include <cstdlib>
#include <curl/curl.h>
 
size_t write_to_file(void *ptr, size_t size, size_t nmemb, FILE *stream)
{
    size_t written = fwrite(ptr, size, nmemb, stream);
    return written;
}
 
int main(void)
{
  CURLcode res;
  CURL *curl;
  curl_slist *recipients = NULL;
  fileBuf_upload_status file_upload_ctx;
  size_t file_size = 0;
 
  file_upload_ctx.lines_read = 0;
  curl = curl_easy_init();
  file_size = read_file(); // возвращает размер сформированного хэдера сообщения
 
  if(curl)
  {
    curl_easy_setopt(curl, CURLOPT_URL, "smtp://smtp.mail.ru:25");
    curl_easy_setopt(curl, CURLOPT_USERNAME, USERNAME);
    curl_easy_setopt(curl, CURLOPT_PASSWORD, PASSWORD);
    curl_easy_setopt(curl, CURLOPT_USE_SSL, (long)CURLUSESSL_ALL);
    curl_easy_setopt(curl, CURLOPT_LOGIN_OPTIONS, "AUTH=LOGIN");
    curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, 0L);
    curl_easy_setopt(curl, CURLOPT_SSL_VERIFYHOST, 0L);
    curl_easy_setopt(curl, CURLOPT_MAIL_FROM, FROM_ADDR);
    recipients = curl_slist_append(recipients, TO_ADDR);
    curl_easy_setopt(curl, CURLOPT_MAIL_RCPT, recipients);
 
    curl_easy_setopt(curl, CURLOPT_INFILESIZE, file_size);
    curl_easy_setopt(curl, CURLOPT_READFUNCTION, fileBuf_source);
    curl_easy_setopt(curl, CURLOPT_READDATA, &file_upload_ctx);
    curl_easy_setopt(curl, CURLOPT_UPLOAD, 1L);
    curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);
 
    FILE *fd = fopen("D:\\buf.txt", "w");
 
    if(!fd) return 5;
 
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_to_file);
    curl_easy_setopt(curl, CURLOPT_WRITEDATA, fd);
    curl_easy_setopt(curl, CURLOPT_USERAGENT, "libcurl-agent/1.0");
    res = curl_easy_perform(curl);
 
    /* check for errors */
    if(res != CURLE_OK)
    {
      fprintf(stderr, "curl_easy_perform() failed: %s\n",
              curl_easy_strerror(res));
    }
 
    /* cleanup curl stuff */
    curl_easy_cleanup(curl);
  }
 
  return 0;
}

curl_easy_setopt(curl, CURLOPT_DEBUGFUNCTION, my_trace);
curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);
curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 1L);
 
curl_easy_setopt(curl, CURLOPT_URL, "smtp://smtp.mail.ru:25");