Man pages matching name curlopt_mail_rcpt in section 3:

CURLOPT_MAIL_RCPT.3.en	debian 10 / libcurl4-doc
CURLOPT_MAIL_RCPT.3.en	debian 11 / libcurl4-doc
CURLOPT_MAIL_RCPT.3.en	debian 12 / libcurl4-doc