easy:SetOpt_Mail_RCPT(rcpts)
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".