Use angle address, as for the rest of the example.

#define SMTPSERVER "smtp.example.com"

#define SMTPPORT ":587" /* it is a colon+port string, but you can set it

                           to "" to use the default port */

#define RECEPIENT "<receipient@example.com>"

#define RECIPIENT "<recipient@example.com>"

#define MAILFROM "<realuser@example.com>"

#define MULTI_PERFORM_HANG_TIMEOUT 60 * 1000

   if(!mcurl)

     return 2;

   rcpt_list = curl_slist_append(rcpt_list, RECEPIENT);

   rcpt_list = curl_slist_append(rcpt_list, RECIPIENT);

   /* more addresses can be added here

      rcpt_list = curl_slist_append(rcpt_list, "others@example.com");

      rcpt_list = curl_slist_append(rcpt_list, "<others@example.com>");

   */

   curl_easy_setopt(curl, CURLOPT_URL, "smtp://" SMTPSERVER SMTPPORT);