This optional parameter allows co-operating agents in a trusted environment to communicate the authentication of individual messages and should only be used by the application program, using libcurl, if the application is itself a mail server acting in such an environment. If the application is operating as such and the AUTH address is not known or is invalid, then an empty string should be used for this parameter.
#CURLOPT_MAIL_RCPT, the address
should not be specified within a pair of angled brackets (<>). However, if an
empty string is used then a pair of brackets will be sent by libcurl as
required by RFC2554.