CURLOPT_MAIL_RCPT (3) - Linux Manuals
CURLOPT_MAIL_RCPT: list of SMTP mail recipients
NAME
CURLOPT_MAIL_RCPT - list of SMTP mail recipientsSYNOPSIS
#include <curl/curl.h>
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_MAIL_RCPT,
struct curl_slist *rcpts);
DESCRIPTION
Pass a pointer to a linked list of recipients to pass to the server in your SMTP mail request. The linked list should be a fully valid list of struct curl_slist structs properly filled in. Use curl_slist_append(3) to create the list and curl_slist_free_all(3) to clean up an entire list.When performing a mail transfer, each recipient should be specified within a pair of angled brackets (<>), however, should you not use an angled bracket as the first character libcurl will assume you provided a single email address and enclose that address within brackets for you.
When performing an address verification (VRFY command), each recipient should be specified as the user name or user name and domain (as per Section 3.5 of RFC5321).
When performing a mailing list expand (EXPN command), each recipient should be specified using the mailing list name, such as "Friends" or "London-Office".